Longman Dictionary of Contemporary English
rest room
noun
EXAMPLES FROM CORPUS
▪ As he finished washing his hands, he heard laughter on the other side of the rest room wall.
▪ We looked over the snacks at Pembina and used the rest room.
▪ When she went into the rest room she sat down for a minute while waiting for Peggy to come out of the lavatory.
Wiktionary
rest room
n. (alternative spelling of restroom English)