Meaning of “reason” - Learner’s Dictionary

reason

noun uk us /ˈriːzən/
WHY [ C ]

A2 the facts about why something happens or why someone does something:

I knew Sam was going - that was the reason I went there.
Is there any particular reason why he doesn't want to come?
He left without giving a reason.
That was the reason for telling her.
RIGHT [ C, U ]

B2 something that makes it right for you to do something:

[ + to do sth ] There is every reason to believe the project will be finished on time.
I think we have reason to be concerned.
ABILITY [ U ]

the ability to think and make good decisions:

By this time he'd lost his powers of reason.
within reason

If something is within reason, it is acceptable and possible:

You can have as much as you like, within reason.

(Definition of “reason noun” from the Cambridge Learner’s Dictionary © Cambridge University Press)