a formal statement of the rules on which a subject of study is based or of ideas that are suggested to explain a fact or event or, more generally, an opinion or explanation

economic theory 经济理论

scientific theory 科学理论

Darwin's theory of evolution 达尔文的进化论

He has a theory that the hole was caused by a meteorite. 他认为那个洞是陨石坠落形成的。

If something is possible in theory, it should be possible, but often it does not happen in that way.

In theory, the journey should take three hours, but in practice it usually takes four because of roadworks. 按理说行程应该只要3小时，但由于道路施工，实际上通常会需要4小时。