theory
/ˈθɪri/
a well-substantiated explanation of some aspect of the natural world; an organized system of accepted knowledge that applies in a variety of circumstances to explain a specific set of phenomena
中文释义：理论；学说
Einstein's theory of relativity revolutionized modern physics.
爱因斯坦的相对论彻底改变了现代物理学。
In theory, the plan should work, but in practice it may fail.
理论上，这个计划应该可行，但实际上可能会失败。