Simple Definition:

A theory is a set of ideas or explanations that help us understand how or why things happen. The ideas in a theory are based on evidence and observations, and they can be tested to see if they are correct. Theories are used in many different fields, such as science, psychology, and sociology.

For example, in the theory of evolution, scientists explain how different species of plants and animals have changed over time. Another example is the theory of relativity, which helps us understand how gravity works in the universe.

Very Simple Definition:

A theory is a way to explain why things happen. Scientists use theories to understand how the world works, like how animals change over time or how things move in space.