Website and network terms explained: a plain-English glossary

Websites and networks come with a lot of jargon. This glossary explains the terms you are most likely to meet, in plain words, with links to longer guides where we have them. Skim it, or come back when a word in an error message or a status page is unfamiliar.

A to C

Availability: The share of time a site or service is working. Usually shown as a percentage. See uptime percentages explained.
Bandwidth: How much data a connection can carry per second. Often confused with speed, but a connection can have plenty of bandwidth and still feel slow if latency is high.
Blocked (in our results): The site answered our check but refused to show us the real page, usually because it filters automated traffic. It does not mean the site is down. See why some sites block automated checks.
Cache: A saved copy of something, kept so it can be reused instead of fetched again. Browsers, DNS resolvers and content delivery networks all keep caches.
CDN (content delivery network): A network of servers around the world that keep copies of a site’s files close to visitors. See what a CDN is.
Certificate: A digital document that proves a website’s identity and enables encrypted connections. See how HTTPS and certificates work.
Cookie: A small piece of data a site stores in your browser, used for things like staying signed in. See clearing cache and cookies.

D to H

DNS (Domain Name System): The system that turns names like example.com into numeric addresses. See what DNS is and why it fails.
Downtime: A period when a site or service is not working for its users.
Failover: Automatically or manually switching to a backup when the main system fails. See how websites stay up.
Firewall: Software or hardware that allows or blocks network traffic according to rules.
Gateway: A server or device that passes traffic between networks or between parts of a system. A “bad gateway” error means one server got an invalid answer from another.
Hop: One step along the route between two computers, usually a router. See how to read a traceroute.
HTTP and HTTPS: The language browsers and servers use. HTTPS is the same thing with encryption added.

I to M

IP address: The numeric address of a device or server on a network. Look up yours with our what is my IP page.
ISP (internet service provider): The company that supplies your internet connection.
Jitter: Variation in delay from one message to the next. See latency, packet loss and jitter.
Latency: The time a message takes to travel to a server and back. Often called ping.
Load balancer: A system that spreads visitors across several servers and avoids unhealthy ones.
MX record: A DNS record that says which servers receive email for a domain. See email not working checklist.

N to R

Name server: A server that holds a domain’s DNS records and answers questions about them.
NXDOMAIN: A DNS answer meaning the name does not exist. See what it means.
Origin server: The real server behind a CDN, where a site’s files and data actually live.
Packet: A small chunk of data. Everything sent over a network is split into packets.
Packet loss: The share of packets that never arrive.
Ping: A simple test that sends a small message and measures the time for the reply. Also used as another word for latency. Try our ping test.
Probe: A machine that runs our checks from a particular location, so we can see a site from many places.
Rate limit: A cap on how many requests are allowed in a given time. See the 429 error.
Redirect: An instruction telling a browser to go to a different address. Test one with our redirect checker.
Resolver: The server that looks up DNS answers for your device.
Router: A device that directs traffic between networks. The box in your home is one.

S to Z

SLA (service level agreement): A promise about service quality, such as a minimum uptime, often with compensation if it is missed. Use our uptime SLA calculator.
SSL and TLS: The technology that encrypts HTTPS connections. TLS is the modern name and SSL the older one people still use.
Status code: The three-digit number a server sends to say how a request went. See HTTP status codes explained.
Timeout: What happens when something takes too long to answer, so the attempt is abandoned. See connection timed out.
TTL (time to live): How long a DNS answer may be reused before it must be checked again.
Uptime: The opposite of downtime: the time a service is working, often shown as a percentage.
VPN: A service that sends your traffic through an encrypted tunnel to another server. See why a VPN can break websites.

Missing a term? Let us know and we will add it.

Check a website now

Is it down for everyone, or just you? Paste an address and we test it from fifteen places.

Open the checker

Related guides

Last updated 2 November 2026. Written by the siteupdown.com team. Spotted a mistake?

← All help guides