Definition from Wiktionary, the free dictionary
See also ónto
- Upon; on top of.
- My cat just jumped onto the keyboard.
- (informal) Aware of.
- The thought-police were onto my plans of World domination.
- (mathematics) 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.
onto (not comparable)
- (mathematics, 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.
- (mathematics): surjective
Derived terms 
- (mathematics): one-to-one and onto
See also 
surjective — see surjective