IPA: /ˈtɛtræd/
KK: /ˈtɛtræd/
A group or set of four items or elements.
In mathematics, a tetrad can refer to a set of four points that define a geometric shape.
Tetrad → It is formed from "tetra-" (from Greek "tettares", meaning four) and "-ad" (a suffix used in English to denote a group or set). The word "tetrad" refers to a group or set of four.
Think of the Greek root "tetra-" meaning four, which helps you remember that a tetrad is a group of four.