Wiktionary
n. 1 (context video games English) A peripheral for video game consoles that increases the number of controller ports, allowing more players to participate. 2 (context uncountable English) A text-entry system for mobile phones where each button has several characters, accessed in turn by repeatedly tapping the button.
Wikipedia
A multitap is a video game console peripheral that increases the number of controller ports available to the player, allowing additional controllers to be used in play, similar to a USB hub. A multitap often takes the form of a box with three or more controller ports which is then connected to a controller port on the console itself.
Mainly sports games have supported multitaps due to the multiplayer aspect of some sport games, though some role-playing video games and first person shooters have taken advantage of multitap support.