1. something that is believed to be true without proof
2. the act of taking on or accepting responsibility, control, or a role
3. the act of taking something for granted or as naturally belonging to oneself

Examples

• Her assumption that he would help proved to be wrong.
• We made the assumption that everyone would arrive on time.
• His assumption of leadership came at a critical moment.
• The new CEO’s assumption of control was smooth and efficient.
• The assumption of power by the military was swift.
• Don’t make assumptions about people based on their appearance.
• The theory is based on several key assumptions about human behavior.