dual

Meanings

1. 1
   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

2. 2
   adj

   having more than one decidedly dissimilar aspects or qualities

   a double (or dual) role for an actor

3. 3
   adj

   Characterized by having two (usually equivalent) components.

   a dual-motor vehicle

4. 4
   adj

   Pertaining to two, pertaining to a pair of.

   dual engine failure

5. 5
   adj

   Pertaining to a grammatical number in certain languages that refers to two of something, such as a pair of shoes.

   Modern Arabic displays a dual number, as did Homeric Greek.

6. 6
   adj

   Being the space of all linear functionals of (some other space).

   Accordingly, a hyperplane in the sample space is dual to a subspace in the variable space.

7. 7
   adj

   Being the dual of some other category; containing the same objects but with source and target reversed for all morphisms.

   Every category is dual to its own dual, so if a statement holds in all categories so does its dual.

8. 8
   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.

Etymology

PIE word *dwóh₁ Borrowed from Latin dualis (“two”), from duo (“two”) + adjective suffix -alis.