speculum

(spĕkˈyə-ləm)

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