(of a function) Assuming each of the values in its codomain; having its range equal to its codomain.
Considered as a function on the real numbers, the exponential function is not onto.
Upon; on top of.
My cat just jumped onto the keyboard.
Aware of.
The thought-police were onto my plans of world domination.
Being an onto function with a codomain of (see below).
The exponential function maps the set of real numbers onto the set of positive real numbers.