In mathematics, approaching indefinitely close, as a line to a curve, but never meeting. See II.n. A straight line whose distance from a curve is less than any assignable quantity, but which does not meet the curve at any finite distance from the origin.n. In zoology, either of two straight lines drawn from the apex to the aperture of the conical part of the phragmocone in the belemnites or fossil dibranchiate cephalopods.