The term "theory" in science doesn't mean something is a guess or an idea. It means something that has been extensively tested and supported by evidence. A theory is a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses.

Here's why Einstein's work is called the General Theory of Relativity, not a hypothesis:

* Extensive Evidence: Einstein's theory of general relativity has been tested and confirmed through numerous experiments and observations. From the bending of light around massive objects to the precise predictions of planetary orbits, the theory has consistently held up.

* Explanatory Power: General relativity goes beyond simply describing observations. It provides a framework for understanding the fundamental nature of gravity, space, and time. It explains phenomena like the expansion of the universe, black holes, and the gravitational lensing of light.

* Predictive Accuracy: General relativity makes accurate predictions about various physical phenomena, which have been repeatedly verified through experiments and observations. This predictive power is a hallmark of a well-established theory.

While the term "hypothesis" implies an initial idea that needs further testing and verification, the General Theory of Relativity has gone far beyond that stage. It has been thoroughly tested, refined, and continues to provide a robust explanation for a wide range of physical phenomena.