indicatrix
/ˌɪndɪˈkeɪtrɪks
/ /ɪnˈdɪkətrɪks/(also optical indicatrix)
noun (plural indicatrices /ɪnˈdɪkətrɪsiːz/) Crystallography
An imaginary ellipsoidal surface whose axes represent the refractive indices of a crystal for light following different directions with respect to the crystal axes.Other contributions to differential geometry which occur in this work include his invention of the ‘Dupin indicatrix’ which gives an indication of the local behaviour of a surface up to the terms of degree two.
Origin
Late 19th century: modern Latin, feminine of Latin indicator 'something that points out'.