A stimulus causes an action or response, like the ringing of your alarm clock if you didn't sleep through it.
Stimulus is a word often used in biology — something that causes a reaction in an organ or cell, for example. In financial and economic contexts, a stimulus may be an incentive: the money you spend on your membership is a stimulus for hitting the gym. Though it can be a synonym for stimulant, they are not always used interchangeably. A stimulant is often a chemical or drug that increases activity — think caffeine. For more than one stimulus, use stimuli, not stimuluses.