1. relaxed and friendly; not following strict rules of behavior or ceremony
2. not official or not following established procedures
3. suitable for everyday use rather than special occasions

Examples

• The meeting had an informal atmosphere with everyone sitting in a circle.
• She wore informal clothes to the picnic.
• We had an informal chat about the project over coffee.
• The restaurant has a very informal dining style.
• His informal speaking style made the audience feel comfortable.
• They reached an informal agreement before the official contract.
• The teacher encouraged informal discussion among students.