What is "tetradic"

Tetradic \Tet*rad"ic\, a. (Chem.) Of or pertaining to a tetrad; possessing or having the characteristics of a tetrad; as, a carbon is a tetradic element.

a. 1 Of or relating to a tetrad. 2 (context of a shape English) Having four-fold symmetry; (non-gloss definition: used especially of the digits 0, 1 and 8.)