vanadium pentoxide

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A highly toxic yellow to red crystalline powder, V₂O₅, used as a catalyst in various organic reactions and as a starting material for other vanadium salts.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun chemistry an orange crystalline powder, V2O5, used as a catalyst in the production of sulphuric acid etc.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any of various oxyacids of vanadium; known mostly in the form of its salts