theory/'θɪərɪ $ 'θɪrɪ/

n. an idea that explains why and how sth happens

n. 〖复-ries/-rɪz/〗 [C; U] 理论：[theory of sth] Darwin's theory of evolution 达尔文的进化论

◇[theory that ...] Nobody can prove the theory that human beings can travel through time. 无人能证明人可以穿越时间的理论。

◇economic theory 经济学理论

in theory 理论上；按理：In theory, their listening skills will improve. 他们的听力技巧按理会有所进步。

常犯错误

✗ The problem exists in the theory, but doesn't actually happen in practice.

✓ The problem exists in theory, but doesn't actually happen in practice. 这个问题理论上是存在的，但实践中不会发生。