from
The Collaborative International Dictionary of English v.0.48
Indicatrix \In`di*ca"trix\, n. [NL.] (Geom. of Three Dimensions)
A certain conic section supposed to be drawn in the tangent
plane to any surface, and used to determine the accidents of
curvature of the surface at the point of contact. The curve
is similar to the intersection of the surface with a parallel
to the tangent plane and indefinitely near it. It is an
ellipse when the curvature is synclastic, and an hyperbola
when the curvature is anticlastic.
[1913 Webster]