A way used by stupid people to define how good a character in a game is, usually used in the Super Smash Bros series of games. Someone using a top-tier character who is then beaten by a low-tier character will immeditely go into super whine mode and claim that the low-tier character 'cheated', because there's no way he/she could have won without cheating. This is bullshit.
Short definiton: A measurement of the cheapness of a character in a game. More cheapness= higher tier.
Player 1: *Chooses Fox*
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.