as·ymp·tote /ˈæsəm(p)ˌtot/
漸近線
asymptote
漸近線
As·ymp·tote n. 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.
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asymptote
n : a straight line that is the limiting value of a curve; can
be considered as tangent at infinity; "the asymptote of
the curve"