dual number

noun2

1. •
   A grammatical number denoting a quantity of exactly two.

   • This is particularly the case with regard to the pronouns of the dual number, which form one of the peculiarities of the language. — 1840
2. •
   An element of an algebra (the algebra of dual numbers) which includes the real numbers and an element ε which satisfies ε ≠ 0 and ε² = 0.