Any of 28 small oblong pieces marked with 0–6 pips in each half, used to play the game of dominoes, in which each piece is laid down to form a line, each player in turn trying to find and lay down a piece with a value matched by that of a piece at either end of the line already formed. This meaning dates from the early 19th century; in an earlier sense, the word meant a loose cloak, worn with a mask for the upper part of the face at masquerades. It is originally recorded in the late 17th century, from French, denoting a hood worn by priests in winter, and is probably ultimately based on Latin dominus ‘lord, master’.
domino theory the theory that a political event in one country will cause similar events in neighbouring countries, like a falling domino causing an entire row of upended dominoes to fall (the domino effect). The term derives from a speech by President Eisenhower in 1954 on the possibility of the spread of communist rule.