1a thing that is accepted as true or as certain to happen, without proof:they made certain assumptions about the market [with clause]:we’re working on the assumption that the time of death was after midnight
2 [mass noun] the action of taking on power or responsibility:the assumption of an active role in regional settlements
3 (Assumption) the reception of the Virgin Mary bodily into heaven. This was formally declared a doctrine of the Roman Catholic Church in 1950.
the feast in honour of the Assumption, celebrated on 15 August.
4 [mass noun] archaic arrogance or presumption.
Middle English(in sense 3): from Old French asompsion or Latin assumptio(n-), from the verb assumere(see assume)
assumption in other Oxford dictionaries
Definition of assumption in the US English dictionary