Definition of “principle” - Learner’s Dictionary

principle

noun uk us /ˈprɪnsəpl/
MORAL RULE [ C, U ]

a rule or belief that influences your behaviour and which is based on what you think is right:

He must be punished - it's a matter of principle.
BASIC IDEA [ C ]

B2 a basic idea or rule that explains how something happens or works:

The organization works on the principle that all members have the same rights.
in principle

If you agree with something in principle, you agree with the idea or plan although you do not know the details or you do not know if it will be possible:

They have approved the changes in principle.
on principle

If you refuse to do something on principle, you refuse to do it because you think it is morally wrong:

She doesn't wear fur on principle.

(Definition of “principle” from the Cambridge Learner's Dictionary © Cambridge University Press)