Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. Two individuals or units regarded as a pair: the motherdaughter 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.
 adj. Made up of two units.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 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.
 Same as dyadic.
 n. A group or association of two chromosomes in certain cells, such as the germcells in certain stages.
 n. In prosody, a group of two lines having different rhythms.
 Noting an axis of twofold symmetry. See symmetry.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 n. two items of the same kind
Etymologies
from The American Heritage® Dictionary of the English Language, 4th Edition
Examples

For example, "the dyad" is the term Murray used for the experiment he performed on Kaczynski and other students.

The rake/victim dyad is organized around the cultural power that men have and women do not.
Notes, "'Mummy, possest': Sadism and Sensibility in Shelley's _Frankenstein_"

The more powerful person in a dyad is the one who has the license to touch.

Yet "dyad" was also the term that he used to describe his nearly fortyyear affair with Christiana Morgan.

Numenius argues that for Pythagoras the dyad was a principle independent of the monad; later thinkers, who tried to derive the dyad from the monad (he does not name names but Eudorus,

The One arise the Ideas and the numbers”: for the dyad is the

Many of these establishments “catered to a particular blackwhite dyad, whether black men seeking white prostitutes or white men seeking black.”

CocoRosie's music lives, breathes, discloses, the authentic continuum of time each of us has lived; it articulates the ambient blur of the motherbaby dyad and sings out the faltering steps and unsurpassed intensities of childhood.
S.X. Rosenstock: CocoRosie Turns It On: Passion Is the Pull of Your Past

Moving to a dyad or even a monad would be an improvement on the status quo.
Rizwan Ladha: Brent Scowcroft's Suggestion on the Nuclear Arsenal

White House Coordinator for Arms Control and Weapons of Mass Destruction Terrorism Gary Samore said that we've reached the level in our forces where further reductions will raise questions about whether we retain the triad or whether we go to a system that only is a dyad.
Rizwan Ladha: Brent Scowcroft's Suggestion on the Nuclear Arsenal
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