What word describes a policeman who is not wearing a uniform?

How do you describe a police officer on duty, who wears casual clothes because he/she doesn't want to disclose his/her identity?

If the policeman is wearing everyday clothes as part of an assignment, if he or she is on duty at the moment and normally wears a uniform but not for this particular job, then I would say undercover. Undercover implies that the policeman is actively trying to hide the fact that they are police.

Plainclothes is usually used to describe officers who don't wear uniforms in general. For example, detectives tend to not wear uniforms or so I gather from TV police shows. I admit I don't have much experience in this field.

If the policeman is simply at home and dressed normally, I would just say that they are not in uniform.

Finally, some other terms you might consider are (definitions from the Free Dictionary):

• mufti

  Civilian dress, especially when worn by one who normally wears a uniform.

• civvies

  Civilian clothes.

In America and the UK the officer would be called plainclothes.

From the Free Online Dictionary:

plainclothes - Wearing civilian clothes while on duty to avoid being identified as police or security