1. explains the meaning of a word or concept
2. establishes the boundaries or limits of something
3. characterizes or describes the essential nature of something

Examples

• The dictionary defines the word clearly.
• She defines success as personal happiness.
• The contract defines the terms of employment.
• This experience defines who I am today.
• The fence defines the property boundary.
• His courage defines his character.
• The law defines what constitutes theft.