1) Protoxide 
Suboxides are a class of oxides wherein the electropositive element is in excess relative to the `normal” oxides. When the electropositive element is a metal, the compounds are sometimes referred to as `metal-rich”. Thus the normal oxide of caesium is Cs2O, which is described as a Cs+ salt of O2−. A suboxide of caesium is Cs11O3, where t...
• (n.) An oxide containing a relatively small amount of oxygen, and less than the normal proportion; as, potassium suboxide, K4O.
Sub·ox'ide noun (Chemistry) An oxide containing a relatively small amount of oxygen, and less than the normal proportion; as, potassium suboxide , K 4 O. 
Type: Term Pronunciation: sŭb-ok′sīd Definitions: 1. That member of a series of oxides containing the least oxygen. Synonyms: protoxide
the oxide of an element that contains the smallest proportion of oxygen.