Rainbow refers to flop board textures that have all different suits on them. In other words, it implies that the flop is offering no flush draw possibilities.
So a K club, 7 spades, 2 hearts would be considered to be a “rainbow flop”. Or one could just say King Seven Deuce rainbow.
If on the turn, the last suit in the deck comes so each card is still a different suit, it is still considered a rainbow board.