is: a straight line which a curve approaches but does not meet. [M1.3]
more precisely is: a straight line related to a curve in such a way that there is at least one direction of travel along the curve in which the shortest distance between them decreases progressively as the distance from the origin to the point becomes very large. [M4.4]
more formally is: a straight line which is the limit of the tangents to a curve as the point at which those tangents touch the curve tends to infinity. [M2.2, M2.3]
Copyright 1997, The Open University