co

"Co" is a versatile little word in English. It works mainly as a prefix (attached to the front of other words) to mean "together" or "jointly", but it also has a couple of standalone uses — as a short form for "company" in business names, and as an informal short form for "commanding officer". Here are the most common uses.

1nounshort for company

In business writing, 'co.' (often with a period) is a short way of writing 'company'. You will see it a lot in the names of businesses, for example 'Smith & Co.' It simply means a group of people working together as a business.

business · Modern, widely used in formal and business writing

She works for Johnson & Co., a well-known trading firm.

The invoice was sent by Taylor & Co. last Monday.

He founded his own co. after years of working for others.

2nounshort for commanding officer

In military contexts, 'CO' (usually written in capital letters) is a short form for 'commanding officer' — the person who is in charge of a military unit. Soldiers use this abbreviation all the time instead of saying the full title.

military · Modern, widely used in military contexts

The soldiers waited for orders from their CO.

The CO decided to move the unit to a safer location.

She was promoted and became the CO of the entire battalion.

3nounshort for conscientious objector

A 'CO' can also mean a 'conscientious objector' — someone who refuses to serve in the military because of their personal, moral, or religious beliefs. This use is less common in everyday conversation but appears in historical and legal discussions.

law / history · Used mainly in historical and legal contexts

During the war, he registered as a CO and was assigned to non-combat duties.

The law made some allowances for COs who refused to carry weapons.

4nounprefix meaning 'together' or 'jointly'

This is the most common use of 'co-'. When you attach it to the front of a word, it means that two or more people are doing something together or sharing a role. Think of it as adding the idea of 'with someone else' to any word. For example, a 'co-author' is someone who writes a book together with another person.

everyday language · Modern, widely used

They are co-authors of the new research paper.

She became the co-founder of a successful tech startup.

The two countries agreed to co-operate on the environmental project.