monoxide
n. C / Un. a chemical substance that has one oxygen atom in each molecule. You usually hear about it when people discuss carbon monoxide, a dangerous gas from car engines or heaters.
n. an oxide containing a single atom of oxygen per molecule. Frequently used in compound forms, most notably in reference to carbon monoxide.
Carbon monoxide is a gas you cannot see or smell.
The landlord installed a detector to warn tenants about dangerous levels of carbon monoxide in the apartment.
While carbon monoxide is the most widely discussed variant due to its toxicity, other monoxides exist as stable chemical compounds in various industrial and geological contexts.
Formed as mon- + oxide; compare the earlier dioxide, pentoxide, and tetroxide.
Often used as part of a compound noun; uncountable when referring to the gas in general, but countable in specific chemical classifications.