What is the meaning of dual?

adjective

• Consisting of or involving two parts or components usually in pairs)
  • usage: "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"
  • synonyms: double, duple
• 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
  • synonyms: double, twofold, two-fold, treble, threefold, three-fold
• A grammatical number category referring to two items or units as opposed to one item (singular) or more than two items (plural. )
  • usage: "ancient Greek had the dual form but it has merged with the plural form in modern Greek"