prin·ci·ple 
/ˈprɪnsəpəl/ noun
plural prin·ciples /ˈprɪnsəpəl/ noun 1 : a moral rule or belief that helps you know what is right and wrong and that influences your actions [count] ▪ He has good principles. ▪ It's against my principles to cheat. [=I believe that cheating is wrong] [noncount] ▪ As a matter of principle, he would not accept the gift. [=he did not feel that it was right to accept the gift] ▪ She refused on principle [=because of her beliefs] to give toy guns to the children. 2 [count] : a basic truth or theory : an idea that forms the basis of something
▪ well-established economic principles ▪ His investment strategy is based on the principle that the stock market offers the best returns for long-term investors. 3 [count] : a law or fact of nature that explains how something works or why something happens
▪ the basic principles of hydraulics/magnetism
▪ well-established economic principles ▪ His investment strategy is based on the principle that the stock market offers the best returns for long-term investors.
▪ the basic principles of hydraulics/magnetism
in principle 1 — used to say that something which has not yet happened or been done should be possible according to what is known ▪ In principle, making the changes should be a simple matter, but there may be problems we haven't thought of. 2 : in a general way and without giving attention to details
▪ They accepted the offer in principle.
▪ They accepted the offer in principle.
Do not confuse principle with principal.
