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 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).