Top definition
Often associated with a program. Add-on refers to an additional feature not included in the inital release, available later usually as a download.
The new Internet Explorer add-on allows you to permanently highlight text in yellow so you can see important information.
by ADB October 12, 2003
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2
A climbing game similar to HORSE in basketball. Two or more climbers will alternately climb up a wall/shelf/boulder one or two holds at a time in attempt to make their move too difficult for their opponent to complete. Each time a mistake is made or a player falls a letter is added. Whoever does not have ADD ON spelled by the time everyone else does is the winner.

This game is common among bouldering cllimbers.
Climber1: "Hey dude wanna play add on?"

Climber2: "Yeah. I hope you can set on a backstep to a pinch hold, bitch!"
by climbtimeBITCH July 02, 2011
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3
A form of vermin native to civilisation that survives by crowding around groups of people and taking up space.
Will: Who are these people?
Maria: Add-ons
Will: Oh
by IsaacHunt December 24, 2015
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4
A strap-on for a female computer. Mispronunciation of hardon.
I got an add-on in PC world.
by Delusional October 08, 2003
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