asymptote
regarding a curve, a line such that the distance between the curve and the line approaches 0 as x→∞ or y→∞
en-GB/ˈæsɪmpˌtəʊt/
en-US/ˈæsəmˌtoʊt/
regarding a curve, a line such that the distance between the curve and the line approaches 0 as x→∞ or y→∞