Squid

[http://www.squid-cache.org/ Squid] is a high-performance proxy server that can also be used as a HTTP accelerator for the webserver. (WMF has switched to using Varnish for HTTP-layer caching in preference to Squid but the term remains.)

See:
 * Squid Reverse Proxy
 * Manual:Squid caching
 * Manual:Varnish caching

and associated configuration variables:
 * UseSquid>Special:MyLanguage/Manual:$wgUseSquid|$wgUseSquid
 * SquidServers>Special:MyLanguage/Manual:$wgSquidServers|$wgSquidServers
 * SquidServersNoPurge>Special:MyLanguage/Manual:$wgSquidServersNoPurge|$wgSquidServersNoPurge

and hook:
 * Special:MyLanguage/Manual:Hooks/TitleSquidURLs