Sentence:  It was only when Einstein's <WSD>theory</WSD> of relativity was published in 1915 that physicists could show that Mercury's "anomaly" was actually because Newton's gravitational <WSD>theory</WSD> was incomplete. 
Meaning of the word marked with <WSD> tags:  A coherent statement or set of ideas that explains observed facts or phenomena and correctly predicts new facts or phenomena not previously observed, or which sets out the laws and principles of something known or observed; a hypothesis confirmed by observation, experiment etc. (from 17th c.)