What is a relation below.
http://www.algebra-class.com/algebraic-relations.html
Function Definition below.
http://www.virtualnerd.com/algebra-1/relations-functions/functions/function-notation/function-conceptual-definition
http://www.studystack.com/matching-192340
http://www.studystack.com/matching-47007
http://www.studystack.com/matching-525386