Real Dictionary

C.O.
countable noun: A soldier's C.O. is his or her commanding officer.

c.o. in American English
abbreviation: care of
abbreviation: cash order

CO in American English
care of
Colorado
abbreviation: Colorado (approved esp. for use with zip code)
the chemical symbol for: cobalt

Co in American English 1
Company

Co in American English 2
cobalt

co in British English
abbreviation for: Commanding Officer
the internet domain name for: a commercial company (used with a country domain name)

Co in British English 1
the chemical symbol for: cobalt

Co in British English 2
abbreviation for: County

Co in British English 3
abbreviation for: Company

co-
prefix: co- is used to form verbs or nouns that refer to people sharing things or doing things together.

co- in American English
together
prefix: var. (before a vowel, h, and gn) of com- ( coadjutor; cohabit; cognate). The prefix co- now productively forms new words from bases beginning with any sound (co-conspirator; co-manage; coseismic), sometimes with the derived sense “ auxiliary, subsidiary”

co- in British English
prefix: together; joint or jointly; mutual or mutually

Co.
Co. is used as an abbreviation for company when it is part of the name of an organization.

Co. in American English
abbreviation: Company

cobalt in British English 1
noun: a brittle hard silvery-white element that is a ferromagnetic metal: occurs principally in cobaltite and smaltite and is widely used in alloys. The radioisotope cobalt-60, with a half-life of 5.3 years, is used in radiotherapy and as a tracer. Symbol: Co;