internal lock; inside key
Refers to a lock or key mechanism on the inside of a door, often a thumb-turn or latch that can be operated from within without a key.
Please lock the inside lock before you go to bed.
The inside lock on this door is broken, so it can only be opened from the outside.
A compound of 内 “inside” and 鍵 “lock, key,” denoting a lock operated or secured from the inside.