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]WordNet (r) 3.0 (2006):
pentoxide n 1: an oxide containing five atoms of oxygen in the molecule