(grammar) Pertaining to grammaticalnumber (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.
(linear algebra) Being the space of all linear functionals of (some other space).
(category theory) Being the dual of (some other category); containing the same objects but with domain and codomain reversed for all functions.