1. an explanation or set of principles that explains how or why something happens
2. an idea or opinion about something that may not be proven
3. the general principles or ideas of a subject rather than its practical application

Examples

• Einstein’s theory of relativity changed our understanding of space and time.
• The detective had a theory about who committed the crime.
• Darwin’s theory of evolution explains how species change over time.
• My theory is that he left early because he was feeling sick.
• She knows music theory but struggles to play the piano.
• In theory, the plan should work perfectly.
• The course covers both theory and practical applications.
• Scientists tested the theory through careful experiments.