/ ˈθɪəri; NAmE ˈθɪri; ˈθiːəri/ noun ( pl. -ies )
[ C , U ] a formal set of ideas that is intended to explain why sth happens or exists :
According to the theory of relativity, nothing can travel faster than light.
[ U ] the principles on which a particular subject is based :
the theory and practice of language teaching
[ C ] theory (that) an opinion or idea that sb believes is true but that is not proved :
I have this theory that most people prefer being at work to being at home.
- in theory
late 16th cent. (denoting a mental scheme of something to be done): via late Latin from Greek theōria contemplation, speculation, from theōros spectator.