In Chapter 1, you solved equations and inequalities in one variable and illustrated the solutions on a number line. An equation or inequality in two variables often uses the variables x and y, and the solutions are the ordered pairs (x, y) that make the equation or inequality true. The ordered-pair solutions can be graphed on a coordinate plane.
A linear equation is an equation whose solutions form a straight line on a graph. An equation that produces a graph that is not a straight line is nonlinear.