of facilities such as telephones or lavatories Unavailable for use by anyone else or indicating unavailability; (`engaged' is a British term for a busy telephone line):
— The lavatory is in use.

synonyms: busy, engaged.

Roget 677: in use; used etc. v.; well-worn, well-trodden.    useful etc. 644; subservient etc. (instrumental) 631.   

Currently being used.