Pronunciation: (dī'ad),[key] —n. 1. a group of two; couple; pair.
2. Biol. a. a secondary morphological unit, consisting of two monads: a chromosome dyad. b. the double chromosomes resulting from the separation of the four chromatids of a tetrad.
3. Chem.an element, atom, or group having a valence of two. Cf. monad, triad (def. 2a).
4. Math.two vectors with no symbol connecting them, usually considered as an operator.
5. Sociol. a. two persons involved in an ongoing relationship or interaction.
b. the relationship or interaction itself.