Definitions
from The American Heritage® Dictionary of the English Language, 5th Edition.
 adjective Having or formed by three planes meeting at a point.
from The Century Dictionary.
 Having three faces; threefaced.
 noun The figure determined by three noncoplanar rays from a point.
from the GNU version of the Collaborative International Dictionary of English.
 adjective (Geom.) Having three sides or faces; thus, a
trihedral angle is a solid angle bounded by three plane angles.
from Wiktionary, Creative Commons Attribution/ShareAlike License.
 adjective geometry Having three plane
faces that meet at a common point
Etymologies
Sorry, no etymologies found.
Examples

There were triple suns that revolved slowly about one another  stange as it may seem  in perfect trihedral symmetry.

There were triple suns that revolved slowly about one another  stange as it may seem  in perfect trihedral symmetry.

The angle between two planes is termed dihedral, between three trihedral, between any number more than three polyhedral.
Encyclopaedia Britannica, 11th Edition, Volume 2, Part 1, Slice 1

Those about the lamina spiralis are certainly necessary  illus. substitution of trihedral for triangular.

If we pass a plane cutting all the edges of a trihedral angle of the poly  hedron (i.e., a cube here), we separate one of the vertices from the rest of the polyhedron.
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