1. A hard ferromagnetic silver-white bivalent or trivalent metallic element; a trace element in plant and animal nutrition
   - cobalt, atomic number 27

1. (mail) to be delivered to (e.g. the address of someone who will then give the delivery to the intended recipient)
   "Please mail this to John, c/o his mother";
   - care of

1. An odourless very poisonous gas that is a product of incomplete combustion of carbon
   "CO detectors are essential in homes with gas appliances";
   - carbon monoxide, carbon monoxide gas
    
2. A state in west central United States in the Rocky Mountains
   - Colorado, Centennial State, Colo.
    
3. One who refuses to serve in the armed forces on grounds of conscience
   "The CO chose to perform alternative civilian service";
   - conscientious objector, conchy [UK, informal], conchie [UK, informal]

1. Together, with, joint

Derived forms: COs, Cos

Type of: American state, contestant, dissenter, dissident, metal, metallic element, monoxide, objector, protester, protestor

Part of: America, cobaltite, 'Merica, smaltite, the States, U.S., U.S.A., United States, United States of America, US, USA

Encyclopedia: Co