dyad

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Two individuals or units regarded as a pair: the mother-daughter dyad.
2. n. Biology One pair of homologous chromosomes resulting from the division of a tetrad during meiosis.
3. n. Chemistry A divalent atom or radical.
4. n. Mathematics A function that draws a correspondence from any vector u to the vector (v·u)w and is denoted vw, where v and w are a fixed pair of vectors and v·u is the scalar product of v and u. For example, if v = (2,3,1), w = (0,-1,4), and u = (a,b,c), then the dyad vw draws a correspondence from u to (2a + 3b + c)w.
5. n. Mathematics A tensor formed from a vector in a vector space and a linear functional on that vector space.
6. adj. Made up of two units.

Century Dictionary and Cyclopedia

1. n. Two units treated as one; a pair; a couple.
2. n. In chem., an elementary substance each of whose atoms, in combining with other atoms or molecules, is equivalent in saturating power to two atoms of hydrogen. For example, oxygen is a dyad as seen in the compound H₂O (water), where one atom of oxygen combines with and saturates two atoms of hydrogen.
3. n. In morphology, a secondary unit of organization, resulting from individuation or integration of an aggregate of monads. See monad.
4. n. In mathematics, an expression signifying the operation of multiplying internally by one vector and then by another.
5. Same as dyadic.
6. n. A group or association of two chromosomes in certain cells, such as the germ-cells in certain stages.
7. n. In prosody, a group of two lines having different rhythms.
8. Noting an axis of twofold symmetry. See symmetry.

Wiktionary

1. n. A set of two different elements.
2. n. music any set of two different pitch classes.
3. n. A pair of things standing in particular relation; dyadic relation.

GNU Webster's 1913

1. n. Two units treated as one; a couple; a pair.
2. n. (Chem.) An element, atom, or radical having a valence or combining power of two.
3. adj. (Chem.) Having a valence or combining power of two; capable of being substituted for, combined with, or replaced by, two atoms of hydrogen. See valence.

WordNet 3.0

1. n. two items of the same kind