a coherent group of general propositions used as principles of explanation for a class of phenomena

Einstein's theory of relativity.

a proposed explanation whose status is still conjectural, in contrast to well-established propositions that are regarded as reporting matters of actual fact.

Mathematics . a body of principles, theorems, or the like, belonging to one subject

number theory.

the branch of a science or art that deals with its principles or methods, as distinguished from its practice

music theory.

a particular conception or view of something to be done or of the method of doing it; a system of rules or principles.

contemplation or speculation.

guess or conjecture.

noun

certainty , fact , proof , reality

noun

approach , argument , assumption , base , basis , code , codification , concept , conditions , conjecture , doctrine , dogma , feeling , formularization , foundation , grounds , guess , guesswork , hunch , idea , ideology , impression , method , outlook , philosophy , plan , position , postulate , premise , presentiment , presumption , proposal , provision , rationale , scheme , shot * , speculation , stab * , supposal , suppose , supposition , surmise , suspicion , system , systemization , theorem , thesis , understanding , hypothesis , postulation , presupposition , analysis , belief , explanation , formula , nation , principle