In mathematics, a curve (also called a curved line in older texts) is, generally speaking, an object similar to a line but that need not be straight. Thus, a curve is a generalization of a line, in that curvature is not necessarily zero. A closed curve is a curve that forms a path whose starting point is also its ending point—that is, a path from any of its points to the same point. Closely related meanings include the graph of a function (as in Phillips curve) and a two-dimensional graph.