A form of competition used in Super Smash Bros Brawl whereby the first person to attain fifty (50) KOs is deemed the winner. Deaths and SDs are disregarded, which often leads to kamikaze strategies designed to maximise KOs.
In the event of two players reaching fifty after the same match, deciding matches are held between ALL competitors. If the deciding match is won by a CPU-controlled player or a non-deciding player (that is, a player who did not reach fifty and initiate the decider), play continues until deciding player wins a match (and hence the fifty). The winner of a decider is determined by overall score (KOs - deaths - SDs), not KOs.
The prefix 'quick' is typically used when only a single fifty is to be played, though this is sometimes said with an element of irony, as a quick fifty between players with a strong rivalry can (and often do) escalate into a series of fifties. Series are played with the score from the previous fifty retained, so losers are tasked with making up the difference in KOs during the next fifty. A series may be substituted for a deciding match, but never follow one.
"Hey mate, are we up for a quick fifty tonight?"
"Argh, I'm so sick of losing fifties this week!"