Definitions:

1. a hypothesis that is taken for granted noun
   • any society is built upon certain assumptions
2. a statement that is assumed to be true and from which a conclusion can be drawn noun
   • Their assumption of my actions were unfounded.
3. the act of assuming or taking for granted noun
   • your assumption that I would agree was unwarranted
4. the act of taking possession of or power over something noun
   • Their assumption of office coincided with the trouble in Cuba.

Synonyms:

• effrontery
• laying claim
• premise
• premiss
• presumption
• presumptuousness
• supposal
• supposition