Final answer:
An onto function is a function in which every element in the codomain has a preimage in the domain.
Step-by-step explanation:
An onto function, also known as a surjective function, is a function in which every element in the codomain has a preimage in the domain. In other words, for every element y in the codomain, there exists an element x in the domain such that f(x) = y.
If a function is onto, it means that its range is equal to its codomain. For example, consider the function f(x) = 2x. The range of this function is all even numbers, which is also the codomain. Therefore, f(x) is an onto function.
On the other hand, if a function is not onto, it means that there are elements in the codomain that do not have a preimage in the domain. For example, consider the function g(x) = x^2. The range of this function is all non-negative real numbers, which is not the same as the codomain (which is all real numbers).