Final answer:
A scientific theory is a well-substantiated explanation of some aspect of the natural world, based on evidence from observation and experimentation.
Step-by-step explanation:
A scientific theory is a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experimentation.
Theories are not based on opinions, guesses, or unverified speculations. They are explanations supported by evidence of how events occur in the natural world.
For example, the theory of biological evolution explains the diversity of life on Earth through natural selection and genetic mutations, and the theory of gravity explains the force that attracts objects towards each other.