What term is defined as an idea accepted as most likely true because it is supported by a lot of eveidence?

The term you're referring to is a "theory." In scientific contexts, a theory is a well-substantiated explanation of some aspect of the natural world that is based on a body of evidence and has withstood rigorous testing and scrutiny. Unlike a hypothesis, which is a tentative explanation, a theory has been extensively validated and is widely accepted within the scientific community.