A stimulus is an event or action that makes something else happen. Other possible answers are trigger or impetus.
Cause.
To "cause" is an action that makes something happen.
"Cause" is the action word that makes something happen.
"Happen" is a verb that refers to the occurrence of an event or situation, while "happening" is a noun that describes an event, situation, or occurrence. So, something can happen, but for it to be a happening, it must be an event or occurrence that is currently taking place.
Something that relies on something else to happen.
Creation; a catalyst makes something happen.
Probability is a field of mathematics that helps determine the likelihood of something happening.
A certain event is any event that will definitely happen. For example, the sunrise tomorrow is something that is certain to happen (although it might be obscured from view by rain, snow, fog or something else); this will certainly happen.
Event 1 makes Event 2 happen.
Event 1 makes Event 2 happen.
It is something that cannot happen.