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Webster 1913 Edition

Noun.

[Pref.

tri-

+ Gr. [GREEK] a seat, base.]

(Geom.)

A figure having three sides.

Noun.

[Gr. three, and side.] A figure having three equal sides.

trihedron ‎(plural trihedrons or trihedra)

(mathematics) A geometric figure composed of three planes meeting at a single vertex.

geometric figure