What is "subtangent"

Subtangent \Sub*tan"gent\, n. (Geom.) The part of the axis contained between the ordinate and tangent drawn to the same point in a curve.

n. (context geometry English) The part of the axis contained between the ordinate and tangent drawn to the same point in a curve.

In geometry, the subtangent and related terms are certain line segments defined using the line tangent to a curve at a given point and the coordinate axes. The terms are somewhat archaic today but were in common use until the early part of the 20th century.