in use in WordNet English dictionary

1. (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)
   "her line is busy"; "receptionists' telephones are always engaged"; "the lavatory is in use"; "kept getting a busy signal"
   busy  engaged  in use 
   
2. currently being used
   "robots are in use throughout industry"
   in use