Irssi can "complete" some words for you - you can write just first few letters of the word and press TAB (or any other key with action word_completion assigned). Then, irssi will choose the most probable matching word and inserts it to the command line. You can press TAB repetitively and irssi will replace the chosen word with another possible words matching to the letters you wrote.

The words chosen usually depend on their context - they can be filenames or command names, but most frequently they are nicks of people on same channels as you are. However, you can have global list of own completions, which apply to all contexts. This command is dedicated to maintaining of such a list.

Without any parameters, /COMPLETION displays list of all user completions.

/COMPLETION [-auto] <prefix> <string> adds completion which gets triggered when you press TAB (or any other key with action word_completion assigned) after writing <prefix> specified and which expands to <string>. When -auto is specified, the completion gets triggered even when you press SPACE or ENTER (or any other key with action check_replaces assigned) after the <prefix>. If there's already some <prefix> completion in the list, it will get replaced.

/COMPLETION -delete <prefix> removes completion of <prefix> from the user completion list.

Note: When -auto is specified for the <prefix> you want to remove you have to quote it like /COMPLETION -delete '<prefix>'. Works with doublequotes aswell.

See also: BIND