1a fundamental truth or proposition that serves as the foundation for a system of belief or behaviour or for a chain of reasoning:the basic principles of justice
(usually principles) a rule or belief governing one’s behaviour:struggling to be true to their own principles [mass noun]:she resigned over a matter of principle
[mass noun] morally correct behaviour and attitudes:a man of principle
2a general scientific theorem or law that has numerous special applications across a wide field.
a natural law forming the basis for the construction or working of a machine:these machines all operate on the same general principle
3a fundamental source or basis of something:the first principle of all things was water
a fundamental quality determining the nature of something:the combination of male and female principles
[with adjective] Chemistry an active or characteristic constituent of a substance, obtained by simple analysis or separation:the active principle of Spanish fly
in principle
as a general idea or plan, although the details are not yet established:the government agreed in principle to a peace plan that included a ceasefire
used to indicate that although something is theoretically possible, in reality it may not actually happen:in principle, the banks are entitled to withdraw these loans when necessary
on principle
because of or in order to demonstrate one’s adherence to a particular belief:he refused, on principle, to pay the fine
