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.
The toilet door said 'Occupied'.
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).