A Unix application program configuration file. On Unix, files named with a leading dot are not normally shown in directory listings. Many programs define one or more dot files in which startup or configuration information may be optionally recorded; a user can customise the program's behaviour by creating the appropriate file in the current or home directory.
Dot files tend to proliferate - with every nontrivial application program defining at least one, a user's home directory can be filled with scores of dot files, without the user really being aware of it. Common examples are .profile, .cshrc, .login, .emacs, .mailrc, .forward, .newsrc, .plan, .rhosts, .sig, .xsession.