Answer:
Inverse of a relation
Reasoning:
the inverse of a function is a full function, this is just a set of pairs. A set of pairs, or relation, where x and y values interchange are inverse of the relation. A one to one function is when a function's inverse is also a function (doesn't have more than one y for each x) which can be tested for on the normal function's graph with a HORIZONTAL line test. A normal parabola isn't one to one. An onto function has to do with every value being used (I don't remember much about them, but once again this isn't a function, but rather a specific set of pairs/data)
Example of inverse of a relation:
Relation: {(0,5), (3,2)}
Inverse: {(5,0), (2,3)}
Example of inverse of a function:
f(x)=5x
f-1(x)=x/5
Example of a one to one function:
f(x)=x+1