bisectrix

noun, plural bi·sec·tri·ces [bahy-sek-trahy-seez] /ˌbaɪ sɛkˈtraɪ siz/.

1. Crystallography. an imaginary line bisecting either the acute angle (acute bisectrix) or the obtuse angle (obtuse bisectrix) of the optic axes of a biaxial crystal.
2. Geometry. a bisector.

noun plural bisectrices (baɪˈsɛktrɪˌsiːz)

1. another name for bisector
2. the bisector of the angle between the optic axes of a crystal