civilian clothing
civilian dress
civilian garb
plain clothes

Even the man in plain clothes started to his feet.

The two men in plain clothes nodded at each other.

One of the men in plain clothes came up to stand beside her.

There were two other county employees with him, both in plain clothes.

There were a few people in plain clothes at the far end, but no reaction from them either.

He'd asked the senior pair to turn up in plain clothes.

And a lot of the agents will be in plain clothes.

Grant followed the man in plain clothes out of the room.

Let her see the police officer in plain clothes enter the room.

They however provided a police service, both in plain clothes and uniform.