## Definitions

### from The American Heritage® Dictionary of the English Language, 4th Edition

• n. Two individuals or units regarded as a pair: the mother-daughter dyad.
• n. Biology One pair of homologous chromosomes resulting from the division of a tetrad during meiosis.
• n. Chemistry A divalent atom or radical.
• 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.
• n. Mathematics A tensor formed from a vector in a vector space and a linear functional on that vector space.

• n. A set of two different elements.
• n. any set of two different pitch classes.
• n. A pair of things standing in particular relation; dyadic relation.

### from the GNU version of the Collaborative International Dictionary of English

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

### from The Century Dictionary and Cyclopedia

• n. Two units treated as one; a pair; a couple.
• 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.
• n. In morphology, a secondary unit of organization, resulting from individuation or integration of an aggregate of monads. See monad.
• n. In mathematics, an expression signifying the operation of multiplying internally by one vector and then by another.
• n. A group or association of two chromosomes in certain cells, such as the germ-cells in certain stages.
• n. In prosody, a group of two lines having different rhythms.
• Noting an axis of twofold symmetry. See symmetry.