Rainbow is a slang term gaining credence as a common descriptor, describing a set of cards with differing suits.
Its most common use is to describe a flop in a game like hold 'em. Rainbow flops are flops which have no two cards of any suit (three different suits) and therefore make a flush draw extremely unlikely (from there, any player with a flush draw has at most a 4% chance of hitting it runner-runner).
Rainbow can also be used to describe a player's hand, if their hand has more than two cards in it. For example, in Omaha a player who receives a rainbow hand is one who has received a hand that has one card from each of the four suits in it. This means the player can not get a flush, no matter what cards appear on the board.