All 6 Uses

factor
in
Kidnapped, by Robert Louis Stevenson
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There would be another factor in his shoes, as fast as spur can drive.†
Chpt 12 *

factor = thing that affects a result or outcome
He's factor to half the gentry in these parts; an auld man, forby: highly respeckit, and he kenned your father.†
Chpt 5
In he steps, and gets papers from King George, to be so-called King's factor on the lands of Appin.†
Chpt 12
He's King's Factor upon both, and from both he has to drive out the tenants; and indeed, Mr. Balfour (to be open with ye), it's my belief that if he escapes the one lot, he'll get his death by the other.†
Chpt 16
"Why, very well said," replies the Factor.†
Chpt 17
I am King's Factor upon several of these estates, and have twelve files of soldiers at my back.†
Chpt 17

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.