Omaha is a variant of Texas Hold'em Poker. Omaha is considered, by some, to be the hardest game of Poker to master. Of all of the different games of Poker, Omaha is for many the hardest to learn to play and the hardest to bluff in. It is played most often at fixed limits, like Texas Hold'em and pot limit.
A slang term for the starting hand of AxKx in hold'em, named for the former professional tennis player and model (a cynical reference to the hand looking better than it plays).