1. A group or set of four
   "The tetrad of elements - earth, air, fire, and water";
   - quaternity
    
2. (biology) a group of four cells or chromosomes formed during meiosis
   "The chromosomes form tetrads during cell division"
    
3. (chemistry) an atom with a valence of four
   "Carbon commonly forms a tetrad"
    
4. (physics) a set of four vectors that form a local basis in spacetime, especially in general relativity
   "The tetrad formalism simplifies many calculations in curved spacetime"

Derived forms: tetrads

Encyclopedia: Tetrad