asymptote
    n 1: a straight line that is the limiting value of a curve; can
         be considered as tangent at infinity; "the asymptote of the
         curve"

Asymptote \As"ymp*tote\ (?; 215), n. [Gr. ? not falling
   together; 'a priv. + ? to fall together; ? with + ? to fall.
   Cf. Symptom.] (Math.)
   A line which approaches nearer to some curve than assignable
   distance, but, though infinitely extended, would never meet
   it. Asymptotes may be straight lines or curves. A rectilinear
   asymptote may be conceived as a tangent to the curve at an
   infinite distance.
   [1913 Webster]