facet

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One of the flat polished surfaces cut on a gemstone or occurring naturally on a crystal.
• noun Anatomy A small, smooth, flat surface, as on a bone or tooth.
• noun Biology One of the lenslike visual units of a compound eye, as of an insect.
• noun One of numerous aspects, as of a subject.

from The Century Dictionary.

• noun A book; especially, a child's book of instruction; a primer.
• To cut a facet or facets upon: as, to facet a diamond.
• noun In the embryo of Pentastomum, the circular thickening left on the detached integument at the site of the dorsal cone after the separation of the embryo from the integument. Also facette. Compare cervical cross.
• noun A little face; a small surface; specifically, in lapidary work, a small polished surface, usually of some geometrical form; one of the many variously shaped segments or faces into which the surface of a gem is broken in order to increase its brilliancy.
• noun In architecture the fillet between the flutings of a column.
• noun In anatomy, a smooth, flat, circumscribed articular surface of bone. See second cut under dorsal.
• noun In entomology, the surface of an ocellus of the compound eye of an insect; also, an ocellus.

from the GNU version of the Collaborative International Dictionary of English.

• noun A little face; a small, plane surface.
• noun (Anat.) A smooth circumscribed surface.
• noun (Arch.) The narrow plane surface between flutings of a column.
• noun (Zoöl.) One of the numerous small eyes which make up the compound eyes of insects and crustaceans.
• transitive verb To cut facets or small faces upon.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any one of the flat surfaces cut into a gem.
• noun One among many similar or related, yet still distinct things.
• noun One of a series of things, such as steps in a project.
• noun anatomy One member of a compound eye, as found in insects and crustaceans.
• noun mathematics A face of codimension 1 of a polytope.
• verb To cut a facet into a gemstone.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a smooth surface (as of a bone or cut gemstone)
• noun a distinct feature or element in a problem