The name is a trademark, usually not calling it "Tetris" makes you measurably safer.
The tetrominos themselves are not protected but the shapes+colours are protected in some way (as original art), as is the official set of gameplay rules, the argument being that the overall combination of these is the "Tetris IP"... although the enforceability of that more often seem to hinge into "I don't have the money to fight an army of lawyers to prove that a gameplay idea isn't protected in yet another jurisdiction".
