n. 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.yad. n. 1. (Mathematics) 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. (Chemistry) an atom or group that has a valency of two.