Asymptote

Courtesy of Merriam Webster’s Online Dictionary

as·ymp·tote
Pronunciation: ?a-s?m(p)-?t?t<br /> Function: noun
Etymology: probably from New Latin *asymptotus, from Greek asympt?tos not meeting, from a- + sympiptein to meet — more at symptom
Date: 1656

: a straight line associated with a curve such that as a point moves along an infinite branch of the curve the distance from the point to the line approaches zero and the slope of the curve at the point approaches the slope of the line.