dy·ad (dd, -d)
1. Two individuals or units regarded as a pair: the mother-daughter dyad.
2. Biology One pair of homologous chromosomes resulting from the division of a tetrad during meiosis.
3. Chemistry A divalent atom or radical.
a. 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.
b. A tensor formed from a vector in a vector space and a linear functional on that vector space.
Made up of two units.
[From Greek duas, duad-, from duo, two; see dwo- in Indo-European roots.]
it always sounds like a wood spirit to me. Oh wait....that's a dyrad.