dual in WordNet English dictionary

1. consisting of or involving two parts or components usually in pairs
   "an egg with a double yolk"; "a double (binary) star"; "double doors"; "dual controls for pilot and copilot"; "duple (or double) time consists of two (or a multiple of two) beats to a measure"
   double  dual  duple 
   
2. having more than one decidedly dissimilar aspects or qualities
   "a double (or dual) role for an actor"; "the office of a clergyman is twofold; public preaching and private influence"- R.W.Emerson; "every episode has its double and treble meaning"-Frederick Harrison
   double  dual  twofold  two-fold  treble  threefold  three-fold 
   
3. a grammatical number category referring to two items or units as opposed to one item (singular) or more than two items (plural)
   "ancient Greek had the dual form but it has merged with the plural form in modern Greek"
   dual