n.

1. 学说；论说；……论[C][（+of）][+that]
   Darwin spent more than twenty years working on his theory of evolution.

   达尔文花了二十余年时间研究他的进化论。

2. 理论；学理，原理[U][C]
   It seems good in theory, but it doesn't work in practice.

   理论上它似乎很不错，但实际上却行不通。

3. 意见；推测，揣度[C][+that]
   My theory is that we'll have a cold winter this year.

   依我看今年冬天会很冷。

   Many scientists accept the theory that the universe is growing larger.

   许多科学家接受这样的看法：宇宙在不断增大。

n.

1. a supposition or a system of ideas intended to explain something, especially one based on general principles independent of the thing to be explained.
   ▸an idea accounting for or justifying something.
2. a set of principles on which an activity is based:
   a theory of education.
   ▸Mathematics  a collection of propositions illustrating the principles of a subject.