Website Monitoring Glossary

Uptime

Uptime is the percentage of time a website or service is reachable and serving correct responses. It is the headline reliability metric and is usually expressed as a number of nines, e.g. 99.9% uptime allows roughly 43 minutes of downtime per month.

Downtime

Downtime is any period in which a website or service is unreachable, returns errors, or fails to meet its agreed performance threshold. It is the inverse of uptime and is the headline figure on most public status pages.

Five Nines(99.999% uptime)

Five nines of availability means 99.999% uptime — a service level that allows no more than 5 minutes 15 seconds of downtime per year. It is the gold standard for telecom and critical infrastructure but is rarely achievable, or economically justified, for typical web SaaS.

SLA(Service Level Agreement)

A service level agreement (SLA) is a contractual commitment between a provider and a customer specifying a measurable level of service — usually uptime percentage — and the financial penalties (service credits) if it is breached.

MTTR(Mean Time To Recovery)

Mean Time To Recovery (MTTR) measures the average time it takes to restore service after an incident begins. It is one of the four DORA metrics and a direct indicator of operational maturity.

Incident

An incident is a confirmed period of degraded or unavailable service that requires human attention. Incidents are usually opened automatically when monitoring detects sustained failures and closed when service is verified restored.

Alert Fatigue

Alert fatigue is the desensitisation that happens when an on-call team receives too many alerts, especially false positives. It causes real incidents to be missed, slows MTTR, and burns out engineers.

Website Monitoring

Website monitoring is the continuous automated checking of a website's availability, performance, security, content, and SEO health. It is the operational discipline that catches problems before customers do — and proves uptime against SLAs.

Synthetic Monitoring

Synthetic monitoring is the practice of running scripted, scheduled checks against a website from controlled probe locations — as opposed to real-user monitoring (RUM), which captures data from actual visitors. It catches issues even when traffic is low.

False Positive

A false positive is a monitoring alert that fires when nothing is actually wrong — a transient network blip, a momentary CPU spike, a single failed probe that recovered immediately. False positives are the primary cause of alert fatigue.

Recovery Time Objective(RTO)

Recovery Time Objective (RTO) is the maximum acceptable time a system can be offline after an incident before the business impact becomes unacceptable. RTO drives the design of failover, backups, and incident response — a 15-minute RTO demands very different infrastructure from a 24-hour RTO.

Recovery Point Objective(RPO)

Recovery Point Objective (RPO) is the maximum amount of data loss (measured in time) a business can tolerate after a disaster. An RPO of 15 minutes means backups or replication must run at least every 15 minutes; an RPO of zero requires synchronous replication.

Mean Time Between Failures(MTBF)

Mean Time Between Failures (MTBF) is the average time a system operates between one incident and the next. MTBF measures how reliable something is; MTTR measures how quickly you recover. Together they define uptime: uptime = MTBF / (MTBF + MTTR).

Service Level Objective(SLO)

SLO means Service Level Objective — the internal reliability target a team commits to, such as 99.9% of homepage requests returning in under 400ms over a rolling 30-day window. SLOs sit inside the SLA envelope and give engineering the numbers to code, alert, and prioritise against.

Service Level Indicator(SLI)

A Service Level Indicator (SLI) is the actual measurement of one aspect of service — usually availability, latency, error rate, or freshness. SLIs are the numbers you compare against SLOs to decide whether the service is meeting its reliability target.

Error Budget

An error budget is the acceptable amount of unreliability inside an SLO — 1 minus the objective. A 99.9% SLO means 0.1% of requests can fail without breaching. That 0.1% is the budget the team can spend on deploys, experiments, and controlled risk.

Status Page

A status page is a public dashboard that displays the current health of a service and the history of past incidents. Status pages set customer expectations during outages, cut inbound support volume, and act as the single source of truth communications teams can point to.

Core Web Vitals(CWV)

Core Web Vitals are a set of three Google-defined metrics that measure real-user experience on a webpage: Largest Contentful Paint (LCP), Interaction to Next Paint (INP), and Cumulative Layout Shift (CLS). They are direct Google ranking signals.

LCP(Largest Contentful Paint)

Largest Contentful Paint (LCP) measures the time from navigation start until the largest visible element on the screen finishes rendering. It is one of three Core Web Vitals and the primary perceived-loading metric.

CLS(Cumulative Layout Shift)

Cumulative Layout Shift (CLS) measures how much visible content unexpectedly moves around the page during loading. It is one of the three Core Web Vitals and quantifies visual stability.

INP(Interaction to Next Paint)

Interaction to Next Paint (INP) measures the longest single-interaction latency a user experiences on a page, from input to the next visual update. It replaced FID as a Core Web Vital in March 2024.

TTFB(Time To First Byte)

Time To First Byte (TTFB) measures how long after a request before the first byte of the response arrives. It captures DNS lookup, TCP/TLS handshake, request routing, and server processing — a comprehensive backend health signal.

Lighthouse

Lighthouse is Google's open-source automated auditing tool for web pages. It produces a 0-100 score across performance, accessibility, best practices, SEO, and PWA, plus actionable recommendations.

Page Speed

Page speed is a colloquial term covering the time a page takes to become visually complete and interactive. It is measured by composite metrics like Core Web Vitals, Speed Index, and Time to Interactive.

CDN(Content Delivery Network)

A Content Delivery Network (CDN) is a globally distributed set of caching servers that serve static assets and (increasingly) dynamic content from a location physically close to each user, dramatically reducing latency.

Real User Monitoring(RUM)

Real User Monitoring (RUM) captures actual page load performance in real visitors' browsers — LCP, INP, CLS, TTFB — instead of synthetic tests. RUM answers 'what do users actually experience?' while synthetic monitoring answers 'is the site up and consistent?'.

SEO(Search Engine Optimization)

Search Engine Optimization (SEO) is the practice of structuring a website's content, code, and reputation so that search engines understand it and rank it for relevant queries. It splits into on-page, technical, and off-page disciplines.

Sitemap

A sitemap is an XML file that lists every URL on a website intended for search engines to index, along with metadata like last modification date and priority. It is the primary mechanism for telling Google and Bing what to crawl.

robots.txt

robots.txt is a plain-text file at the root of a website that tells well-behaved web crawlers which URLs they can and cannot access. It is the first file any reputable bot fetches when visiting a domain.

Canonical Tag

A canonical tag is an HTML link element (`<link rel="canonical" href="...">`) that tells search engines which URL is the master version when the same content is reachable at multiple URLs. It consolidates ranking signals onto one page.

Indexing

Indexing is the process by which a search engine adds a page to its searchable database after crawling it. A page must be indexed before it can rank for any query — and indexing is no longer guaranteed for low-quality content.

IndexNow

IndexNow is an open protocol jointly developed by Microsoft and Yandex that lets websites instantly notify search engines when content is added, updated, or deleted — replacing slow scheduled crawling with sub-second push notifications.

Schema Markup

Schema markup is structured data added to a webpage in JSON-LD format that describes the page's content in machine-readable terms. It powers rich results in search, AI overviews, and LLM citation graphs.

SSL/TLS(Secure Sockets Layer / Transport Layer Security)

SSL/TLS is the cryptographic protocol that secures HTTPS connections. It encrypts traffic between browser and server, authenticates the server's identity via a certificate, and ensures message integrity. SSL is the deprecated name; modern deployments use TLS 1.2 or TLS 1.3.

CSP(Content Security Policy)

Content Security Policy (CSP) is an HTTP response header that restricts which sources a browser may load scripts, styles, images, and other resources from. It is the primary defence against cross-site scripting (XSS) attacks.

HSTS(HTTP Strict Transport Security)

HTTP Strict Transport Security (HSTS) is an HTTP response header that tells browsers to access the site only over HTTPS for a specified duration. It prevents protocol-downgrade attacks and accidental insecure links.

Security Headers

Security headers are HTTP response headers that instruct browsers to enforce additional defences against common web attacks. Standard set: HSTS, CSP, X-Content-Type-Options, X-Frame-Options, Referrer-Policy, Permissions-Policy.

DMARC(Domain-based Message Authentication, Reporting and Conformance)

DMARC is an email authentication protocol that lets a domain owner publish a policy specifying how mailbox providers should handle messages that fail SPF or DKIM checks. It is the keystone defence against email spoofing of your domain.

SPF(Sender Policy Framework)

Sender Policy Framework (SPF) is an email authentication standard that publishes the list of IP addresses authorised to send mail on behalf of a domain. Mailbox providers reject mail from unlisted IPs.

DKIM(DomainKeys Identified Mail)

DomainKeys Identified Mail (DKIM) is an email authentication standard that adds a cryptographic signature to every outgoing message, allowing the receiver to verify the message was authorised by the claimed sender domain and not modified in transit.

DNS(Domain Name System)

The Domain Name System (DNS) is the distributed database that translates human-readable domain names like lemwatch.com into the IP addresses computers use to route traffic. It is the address book of the internet.

Domain Expiry

Domain expiry is the date a domain registration lapses. After expiry, the domain enters a grace period; after that, it can be re-registered by anyone — and recovery is usually impossible.

RDAP(Registration Data Access Protocol)

Registration Data Access Protocol (RDAP) is the modern replacement for WHOIS. It returns structured JSON data about domain registration — registrar, expiry date, status flags, contact (where permitted) — over a secure HTTPS API.

Broken Link

A broken link is a hyperlink that no longer points to a working destination — typically returning a 404 (not found), 410 (gone), 500 (server error), or DNS-resolution failure. Broken links degrade user experience and SEO.

Visual Regression

A visual regression is an unintended visual change to a webpage — a button moved, text colour changed, a section disappeared, an image broken — caused by a code or content update. Visual regression testing automates the detection of these changes.

Change Detection

Change detection is the practice of monitoring a webpage for content modifications and alerting when something changes. It applies to text, images, prices, structured data, and full-page screenshots.

Money Page

A money page is a webpage directly responsible for revenue — the homepage, a pricing page, a checkout flow, a high-converting landing page. It is the page where outages and content errors translate into immediate lost revenue.