speculum

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

• noun A mirror or polished metal plate used as a reflector in optical instruments.
• noun An instrument for dilating the opening of a body cavity for medical examination.
• noun A bright, often iridescent patch of color on the wings of certain birds, especially ducks.
• noun A transparent spot in the wings of some butterflies or moths.

from The Century Dictionary.

• noun In astrology, a table exhibiting at one view the latitudes, destinations, semi-arcs, etc., of the planets in a nativity.
• noun Something to look into or from; specifically, a mirror or looking-glass.
• noun An attachment to or part of an optical instrument, as a reflecting telescope, having a brightly polished surface for the reflection of objects.
• noun In ornithology:
• noun An ocellus or eye-spot, as of a peacock's tail. See ocellus, 4.
• noun The mirror of a wing, a specially colored area on some of the flight-feathers.
• noun In anatomy, the septum lucidum of the brain. See cut under corpus.
• noun In medicine and surgery, an instrument used for rendering a part accessible to observation, especially by opening or enlarging an orifice.
• noun A lookout; a place to spy from.

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

• noun A mirror, or looking-glass; especially, a metal mirror, as in Greek and Roman archæology.
• noun A reflector of polished metal, especially one used in reflecting telescopes. See Speculum metal, below.
• noun (Surg.) An instrument for dilating certain passages of the body, and throwing light within them, thus facilitating examination or surgical operations.
• noun (Zoöl.) A bright and lustrous patch of color found on the wings of ducks and some other birds. It is usually situated on the distal portions of the secondary quills, and is much more brilliant in the adult male than in the female.
• noun a hard, brittle alloy used for making the reflectors of telescopes and other instruments, usually consisting of copper and tin in various proportions, one of the best being that in which there are 126.4 parts of copper to 58.9 parts of tin, with sometimes a small proportion of arsenic, antimony, or zinc added to improve the whiteness.

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

• noun medicine A medical instrument used during an examination to dilate an orifice.
• noun A mirror.

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

• noun a mirror (especially one made of polished metal) for use in an optical instrument
• noun a medical instrument for dilating a bodily passage or cavity in order to examine the interior