Short definiton: A measurement of the cheapness of a character in a game. More cheapness= higher tier.
Player 2: *Chooses Kirby*
Player 1: LOL Kirby is low-tier you're gonna get owned
The winner is.... KIRBY
Player 1: OMG U FUCKING CHEATER THERES NO WAY A KIRBY COULD BEAT A FOX!!!!shiftone
Player 2: Dude... Stop whining, you got owned.