Website monitoring glossary
The words that come up when you keep a website online, each defined in a sentence or two, with a worked example and common questions.
Uptime
Uptime is the share of time a website or service is reachable and working, usually shown as a percentage of a period such as a month.
Downtime
Downtime is time when a website or service can't be used: it doesn't answer, answers with an error, or is too slow to be usable.
SLA (service level agreement)
An SLA is a written promise from a provider about the level of service, such as 99.9% monthly uptime, and what you get, often a service credit, when it is missed.
SLO (service level objective)
An SLO is an internal reliability target, such as 99.95% of checks succeeding over 30 days, usually set stricter than any SLA so problems show up before a promise is broken.
Incident
An incident is an unplanned event that interrupts a service or lowers its quality, from a full outage to a slow checkout. It lasts until the service is back to normal.
Status page
A status page is a public web page that shows whether a service is working right now, its recent history, and updates on any incident in progress.
SSL certificate expiry
SSL certificate expiry is the date after which a site's TLS (SSL) certificate stops being valid. From then on, browsers warn visitors that the connection isn't secure.
Certificate chain
A certificate chain links a site's own certificate, through one or more intermediate certificates, to a root certificate the browser already trusts.
DNS propagation
DNS propagation is the time a change to a domain's DNS records takes to reach everyone, as resolvers around the world drop their cached copies of the old records.
TTFB (time to first byte)
TTFB is how long a browser waits, after asking for a page, before the first byte of the answer arrives. It includes the DNS lookup, connecting, TLS setup and the server's work.
HTTP status code
An HTTP status code is the three-digit number a server sends with every response to say how the request went: 2xx success, 3xx redirect, 4xx client error, 5xx server error.
Redirect chain
A redirect chain is a series of redirects one after another, such as http to https to www to a new path, before a browser reaches the final page.
Synthetic monitoring
Synthetic monitoring checks a website by sending scripted requests on a schedule from outside, instead of waiting for real visitors to run into a problem.
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