A short sequence of face turns, which is generally fast and easy to execute. The Sexy Move is an example of a trigger. When repeated enough times on a solved cube, the cube will revert to its solved position. The number of moves of most triggers is 4 moves.
A trigger is not usually considered an algorithm, as it does not by itself create the desired change in the puzzle. Instead, triggers are combined to create algorithms. One exception to this is the sexy method, which uses the sexy move as its main algorithm.
When notating algorithms, the triggers are usually enclosed in parenthesis. This aids in memorization, and also in execution. For example, the OLL algorithm #2 is written as F (R U R' U') (R U R' U') F', and thought of as a conjugate which encloses two sexy moves. Each sexy move, with practice, will require one conscious thought, with the four turns carried out via muscle memory. Hence, the execution is as easy as pulling a trigger.