What is "tetra-"

Tetra- \Tet"ra-\ [Gr. te`tra-, from te`sares, te`ttares, four. See Four.]

1. A combining form or prefix signifying four, as in tetrabasic, tetrapetalous.

2. (Chem.) A combining form (also used adjectively) denoting four proportional or combining parts of the substance or ingredient denoted by the term to which it is prefixed, as in tetra-chloride, tetroxide.

before vowels tet-, word-forming element meaning "four," from Greek tetra-, combining form of tettares (Attic), tessares "the numeral four" (see four).

pre. Forming compound words having the sense of ‘four’.