dual >> /ˈd(j)uəl/
Definition: [noun] A dual form of a word.
Definition: [noun] A theorem, expression, etc., that is dual to another.
Example: In fact this theorem is simply the dual of Pascal's theorem which was proved in 1639.
Definition: [adjective] Consisting of two parts, elements, or aspects.
Example: their dual role at work and home
Definition: [adjective] (of a theorem, expression, etc.) related to another by the interchange of particular pairs of terms, such as “point” and “line.”.