1. (

• carbon_monoxide ,
• carbon_monoxide_gas ,
• CO

) an odorless very poisonous gas that is a product of incomplete combustion of carbon ;

2. (

• cobalt ,
• Co ,
• atomic_number_ ,

) a hard ferromagnetic silver-white bivalent or trivalent metallic element; a trace element in plant and animal nutrition ;

3. (

• conscientious_objector ,
• CO

) one who refuses to serve in the armed forces on grounds of conscience ;

4. (

• Colorado ,
• Centennial_State ,
• CO ,
• Colo.

) a state in west central United States in the Rocky Mountains ;