subtangent
/ˈsʌbˌtan(d)ʒ(ə)nt/noun
In Cartesian coordinates: the part of the x-axis lying between the two points where it is cut by the tangent at a given point on a curve and by the perpendicular drawn from that point to the x-axis.
Origin
Early 18th century; earliest use found in John Harris (c1666–1719), writer and lecturer on science. From post-classical Latin subtangent-, subtangens from sub- + tangens.