All 3 Uses of

factor
in
The Adventures of Sherlock Holmes

This Godfrey Norton was evidently an important factor in the matter.†
Chpt 1 *

factor = thing that affects a result or outcome

But for the trained reasoner to admit such intrusions into his own delicate and finely adjusted temperament was to introduce a distracting factor which might throw a doubt upon all his mental results.†
Chpt 1

"It seems to me that I have done you full justice in the matter," I remarked with some coldness, for I was repelled by the egotism which I had more than once observed to be a strong factor in my friend's singular character.†
Chpt 12

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)
meaning too rare to warrant focus:

See a comprehensive dictionary for other meanings. Less common meanings include one that has to do with units of measurement. More specialized meanings are used in fields including mathematics, business, finance, biology, and grammar.