1. carbon monoxide, carbon monoxide gas, co

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

• carbon monoxide
• carbon monoxide gas

2. atomic number 27, co, cobalt

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

• atomic number 27
• cobalt

3. co, conscientious objector

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

• conscientious objector

4. centennial state, co, colorado

a state in west central United States in the Rocky Mountains

• centennial state
• colorado