Common Core State Standard: 8.F.1 

Grade  8 
Subject  MATH 
Domain  Functions 
Theme  Define, evaluate, and compare functions 
Standard  Understand that a function is a rule that assigns to each input exactly one output. The graph of a function is the set of ordered pairs consisting of an input and the corresponding output. (Function notation is not required in Grade 8.) 
Reference URL  http://www.corestandards.org/Math/Content/8/F 