A bathroom with a door that locks. These bathrooms are not meant to contain more than one person in them at a time. Even if one person is only washing his/her hands it is considered taboo to enter a lock-door bathroom.
Commonly found in convenience stores, and fast food restaurants where space is limited. These can be found in a variety of modus operandi including, but not limited to, unisex, and handicapped.
"Brandon, don't go in that bathroom. It's a lock-door bathroom and someone is already using it. You don't 2-person in a lock-door bathroom."