1 countable something that you think is true although you have no definite proof
A. assumption that
B. assumption about
2 uncountable formal when someone starts to have control or power
A. assumption of
1. A lot of people make the assumption that poverty only exists in the Third World.
My calculations were based on the assumption that house prices would remain steady.
We are working on the assumption that the conference will take place as planned.
B. People make a lot of assumptions about me.
underlying assumption (=a belief that is used as the basis for an idea, but which may not be correct)