Theory (noun)

A set of principles or explanations used to describe or understand a particular subject or phenomenon.

Origin:

From the greek theoria "contemplation".

Examples:

1. The theory of evolution.
2. He has a theory about why the stock market fluctuates.
3. Her theory of the case was proven to be correct.
4. The theory of gravity was first proposed by Isaac Newton.
5. His theory of the universe was later disproved by Einstein's theory of relativity.