Full Idea
A 'surjective' function is 'onto' - the whole of the output set results from the function being applied to elements of the original set.
Gist of Idea
A 'surjective' ('onto') function creates every element of the output set
Source
Peter Smith (Intro to Gödel's Theorems [2007], 02.1)
Book Reference
Smith,Peter: 'An Introduction to Gödel's Theorems' [CUP 2007], p.8