A domino is a $2$-omino:
The plural of domino is generally considered to be dominoes, but the form dominos can also be seen.
An older meaning of the word domino is a carnival costume featuring a black robe and a mask, often white, that covers the eyes.
It is supposed that the name of the game dominoes may have derived from the carnival costume meaning.
From there evolved the usage in the context of polyominoes.