Search domain is a network tool used in *nix operating systems such as Linux or OS X.
In Linux it can be defined by editing the ifcfg file corresponding to the network. In MacOS the setting is located under the DNS tab in Mac OS X, next to DNS server settings. A similar setting in Microsoft Windows is the Connection-specific DNS Suffix.
Functionality
When looking up a bare name in DNS, the network stack will add the search domains to it to form fully qualified domain names, and look up those as well. For example, if the search domain was wikipedia.org, typing "en" in the browser will direct the user to "en.wikipedia.org". Some ISPs add their own search domains via DHCP settings, similar to how they add DNS servers and other networking information; if this is undesired, the user can change this setting to ".local".