All 4 Uses

factor
in
Cujo
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He had known Donna was cooling it, but she had struck him as a woman who could be manipulated with no great difficulty, at least for a while, by a combination of psychological and sexual factors.†

factors = things that affect a result or outcome
And she hated him as well as herself for making her feel so badly ashamed, because she didn't believe she was responsible for the factors leading up to the final decision — if there really had been a decision.†
That had really been the deciding factor.†

factor = thing that affects a result or outcome
Free will was not a factor.†
*

Definitions:

(1) (factor as in: It was the deciding factor.)
something that affects a result or outcome

You also may encounter x-factor or x factor--meaning "the most important thing that influences a result or outcome."
(2) (factor as in: factor it into your thinking)
include consideration of
(3) (meaning too rare to warrant focus)
There are many specialized meanings in fields such as mathematics, business, finance, biology, and grammar.