assertion (plural assertions) 1  The act of asserting, or that which is asserted; positive declaration or averment; affirmation; statement asserted; position advanced. You're a man of strong assertions! 2  A statement or declaration which lacks support or evidence. That's just a bare assertion. Maintenance; vindication the assertion of one's rights or prerogatives. (computing) A statement in a program asserting a condition expected to be true at a particular point, used in debugging.