Merriam-Webster Collegiate Dictionary - asymptote

 noun  Etymology: probably from New Latin *asymptotus, from Greek asymptotos 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  • asymptotic adjective  • asymptotically adverb