Cause

An event that increases or decreases the probability that a second event will occur. For example, smoking increases the probability that a person will develop lung cancer.

As this example shows, a cause is not something that always leads to the other event (not everyone who smokes develops lung cancer). Furthermore, a cause is not something that must occur in order for the other event to occur (not everyone who develops lung cancer has smoked).

See: causal relationship
See: multifactorial causation