The act of taking to or upon oneself: assumption of an obligation. The act of taking possession or asserting a claim: assumption of command. The act of taking for granted: assumption of a false theory. Something taken for granted or accepted as true without proof; a supposition: a valid assumption. Presumption; arrogance. Logic A minor premise. Christianity The taking up of the Virgin Mary into heaven in body and soul after her death. A feast celebrating this event. August 15, the day on which this feast is observed.