adj

• (grammar) 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."
• (mathematics, physics) Exhibiting duality.
• (linear algebra) 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."
• (category theory) 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."

noun

• (geometry) 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."
• (grammar) The dual number.
• (mathematics) 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.
• (wrestling) A head-to-head match or meet between two teams, such as two high schools or colleges.
  "Wrestling duals and meets take place at the Wisconsin Field House. There are no advance single event ticket sales."

verb

• (transitive) To convert from single to dual; specifically, to convert a single-carriageway road to a dual carriageway.
  "I have to declare an interest and I do so with some ambivalence because if the road is dualled it is likely to take half of my front garden."