| n. | 1. | The quality or condition of being two or twofold; dual character or usage. |
| Noun | 1. | duality - being twofold; a classification into two opposed parts or subclasses; "the dichotomy between eastern and western culture" Synonyms: dichotomy |
| 2. | duality - (physics) the property of matter and electromagnetic radiation that is characterized by the fact that some properties can be explained best by wave theory and others by particle theory Synonyms: wave-particle duality | |
| 3. | duality - (geometry) the interchangeability of the roles of points and planes in the theorems of projective geometry |