A key recognized by X that you can use to enter a whole range of Unicode characters with mnemonic key sequences. For example, compose-singlequote-e to enter an “é” (e-acute) character.

By default this function is not assigned to any key. You can tell the X server what key to use for this function with a command of the form

setxkbmap -option compose:keyname

for example

setxkbmap -option compose:caps

to assign it to the caps-lock key. (Do you use caps-lock for anything else? I don’t.) GUI environments like GNOME and KDE probably provide some way you can configure this automatically every time you login.

The key sequences you can use depend on your locale, and are defined in the file

/usr/share/X11/locale/localename/Compose

To get the largest choice of characters, make sure you select a UTF-8-encoded locale.