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noun
1.
a group of two; couple; pair.
2.
Biology.
1. a secondary morphological unit, consisting of two monads:
2. the double chromosomes resulting from the separation of the four chromatids of a tetrad.
3.
Chemistry. an element, atom, or group having a valence of two.
4.
Mathematics. two vectors with no symbol connecting them, usually considered as an operator.
5.
Sociology.
1. two persons involved in an ongoing relationship or interaction.
2. the relationship or interaction itself.
6.
1665-1675
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2017.
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Examples from the Web for dyad
Historical Examples
• The respondent was asked, Do you know that every dyad is even?

George Grote
• Some dyad was then produced, which the respondent did not know to be a dyad; accordingly he did not know it to be even.

George Grote
• When you say a line, do you mean a dyad in length Form in Matter?

George Grote
• For either the dyad is not One; or else the monads included therein are not monads ἐντελεχείᾳ (a. 14).

George Grote
• There being six pairs of parallel edges on an octahedron, there are consequently six dyad axes of symmetry.

• They are parallel to the edges of the cube, and in the different classes coincide either with tetrad or dyad axes of symmetry.

• The other elements of symmetry (six dodecahedral planes and six dyad axes) are the same as in the last class.

• There are five axes of symmetry, one tetrad and two pairs of dyad, each perpendicular to a plane of symmetry.

• Here there is only one dyad axis in which two planes of symmetry intersect.

• Here there is a single plane of symmetry perpendicular to which is a dyad axis; there is also a centre of symmetry.

/ˈdaɪæd/
noun
1.
(maths) an operator that is the unspecified product of two vectors. It can operate on a vector to produce either a scalar or vector product
2.
an atom or group that has a valency of two
3.
a group of two; couple
Word Origin
C17: from Late Latin dyas, from Greek duas two, a pair
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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n.

1. Two individuals or units regarded as a pair, such as a mother and a daughter.

2. A divalent atom or radical.

3. One pair of homologous chromosomes resulting from the division of a tetrad during meiosis.

The American Heritage® Stedman's Medical Dictionary