I. ˈte.ˌtrad noun
( -s )
Etymology: Greek tetrad-, tetras group of four, four; akin to Greek tettares, tessares four — more at four
1. : a group or arrangement of four : something composed of four parts : quaternion: as
a. : a tetravalent element, atom, or radical
b. : a group of four cells arranged usually in the form of a tetrahedron and produced by the successive divisions of a mother cell — compare spore mother cell
c. : an arrangement of chromosomes by fours in the first meiotic prophase due to precocious longitudinal splitting of paired homologous chromosomes
2. : tetralogy 3
II. adjective
: fourfold — used especially of an axis of symmetry
a tetrad axis requires that every aspect of the crystal be repeated at 90 degree intervals during rotation about the axis