domino theory
1. The political theory that, if one country or area adopts communism, then the surrounding ones will as well. The domino theory was a major concern as the Soviet Union expanded its power after World War II.
2. By extension, the idea that an event in one country will result in similar events in the surrounding countries. The string of revolutions that occurred in neighboring countries during the Arab Spring was a great example of domino theory in action.
domino effect
A phenomenon in which an initial event causes a series of other related events to occur, much like the sequence seen in toppling dominos; a chain reaction. The convenience store's decision to stop selling tobacco products caused a domino effect throughout the industry, leading other chains to quickly follow suit. When I took one book out of the bookcase, it caused a domino effect, causing all of the others to topple over.