dyad

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Two individuals or units regarded as a pair.
• noun Biology One pair of homologous chromosomes resulting from the division of a tetrad during meiosis.
• noun 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.
• noun A tensor formed from a vector in a vector space and a linear functional on that vector space.
• adjective Made up of two units.

from The Century Dictionary.

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

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

• adjective (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.
• noun Two units treated as one; a couple; a pair.
• noun (Chem.) An element, atom, or radical having a valence or combining power of two.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun two items of the same kind