The Collaborative International Dictionary of English v.0.48:
Tetra- \Tet"ra-\ [Gr. te`tra-, from te`sares, te`ttares, four.
1. A combining form or prefix signifying four, as in
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.
WordNet (r) 3.0 (2006):
n 1: brightly colored tropical freshwater fishes