pentoxide

from WordNet (r) 3.0 (2006)

pentoxide
    n 1: an oxide containing five atoms of oxygen in the molecule

from The Collaborative International Dictionary of English v.0.48

Pentoxide \Pen*tox"ide\, n. [Penta- + oxide.] (Chem.)
   An oxide containing five atoms of oxygen in each molecule;
   as, phosphorus pentoxide, {P2O5}.
   [1913 Webster]