doublet

(dŭbˈlĭt)

1. A close-fitting jacket, with or without sleeves, worn by European men between the 15th and 17th centuries.
2. a. A pair of similar or identical things.
   b. A member of such a pair.
   c. Physics A multiplet with two members.
3. Linguistics One of two words derived from the same historical source by different routes of transmission, such as skirt from Scandinavian and shirt from English.
4. An imitation gem composed of two parts, as of an inferior stone layered beneath a precious gem.
5. doublets Games A throw of two dice in which the same number of dots appears on the upper face of each.