Princeton's WordNet

1. double, dual, duple(adj)

   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"

2. double, dual, twofold, two-fold, treble, threefold, three-fold(adj)

   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

3. dual(adj)

   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"

Wiktionary

1. dual(Noun)

   Of an item that is one of a pair, the other item in the pair.

2. dual(Noun)

   Of a regular polyhedron with V vertices and F faces, the regular polyhedron having F vertices and V faces.

   The octahedron is the dual of the cube.

3. dual(Noun)

   dual number The grammatical number of a noun marking two of something (as in singular, dual, plural), sometimes referring to two of anything (a couple of, exactly two of), or a chirality-marked pair (as in left and right, as with gloves or shoes) or in some languages as a discourse marker, "between you and me". A few languages display trial number.

4. dual(Noun)

   Of a vector in an inner product space, the linear functional corresponding to taking the inner product with that vector. The set of all duals is a vector space called the dual space.

5. dual(Adjective)

   Characterized by having two (usually equivalent) components.

6. dual(Adjective)

   Double.

   dual-headed computer

7. dual(Adjective)

   Pertaining to grammatical number (as in singular and plural), referring to two of something, such as a pair of shoes, in the context of the singular, plural and in some languages, trial grammatical number. Modern Arabic displays a dual number, as did Homeric Greek.

Origin: dualis, from duo, + adjective suffix -alis

Webster Dictionary

1. Dual(adj)

   expressing, or consisting of, the number two; belonging to two; as, the dual number of nouns, etc. , in Greek

• dubbelAfrikaans
• مُزْدَوِجArabic
• двоенBulgarian
• duploPortuguese (BR)
• dvojíCzech
• doppeltGerman
• dobbeltDanish
• διπλόςGreek
• dobleSpanish
• kahekordneEstonian
• دوگانه؛ دوجانبهFarsi
• kaksois-Finnish
• doubleFrench
• זוּגִי, כָּפוּלHebrew
• द्वि, द्वय, दोहराHindi
• dvostruki, dupliCroatian
• kettősHungarian
• gandaIndonesian
• tvöfaldurIcelandic
• doppioItalian
• ２重のJapanese
• 2개의Korean
• dvigubas, dvejopasLithuanian
• divkāršs; dubultsLatvian
• dwiMalay
• tweevoudigDutch
• dobbelt, to-, tve-Norwegian
• dwojaki, podwójnyPolish
• دوگانه؛ دوجانبهPersian
• دوګانه، دوه جانبهPashto
• duploPortuguese
• dubluRomanian
• двойнойRussian
• dvojakýSlovak
• dvojenSlovenian
• dvostrukiSerbian
• dubbel, tvåfaldigSwedish
• เป็นคู่Thai
• ikili,çiftTurkish
• 成雙的，二重的Chinese (Trad.)
• подвійний; спільнийUkrainian
• دگناUrdu
• đôiVietnamese
• 双的，二重的Chinese (Simp.)