expression, noun which may take the genitive case particle 'no'
in use; occupied; engaged
Indicates that something is currently being used, often seen on signs for toilets, meeting rooms, or equipment.
トイレのドアに「使用中」と書いてあった。
The toilet door said 'Occupied'.
この電話は使用中です。
This phone is in use.
使用中 is a general term for something being 'in use' (a phone line, a restroom, etc.), while 話し中 specifically describes a phone line being busy because someone is talking.
使用中 means 'currently in use' (e.g. a restroom occupied now), whereas 使用済み means 'already used/finished being used', so the two describe opposite stages of use.
Compound of 使用 (use) and 中 (in the state of).