gonio-

Angle.

Greek gōnia, angle.

A goniometer is an instrument that measures angles, especially those between the faces of crystals; goniometry is the study of such angular relationships. In medicine, a gonioscope is an optical instrument used for viewing otherwise hidden structures within the eye; goniotomy (Greek ‑tomia, cutting) is an eye operation for congenital glaucoma in which a incision is made at the junction of the cornea and sclera within the eye.