Common Core State Standard: 8.F.3 

Grade  8 
Subject  MATH 
Domain  Functions 
Theme  Define, evaluate, and compare functions 
Standard  Interpret the equation y = mx + b as defining a linear function, whose graph is a straight line; give examples of functions that are not linear. For example, the function A = s^2 giving the area of a square as a function of its side length is not linear because its graph contains the points (1,1), (2,4) and (3,9), which are not on a straight line. 
Reference URL  http://www.corestandards.org/Math/Content/8/F 