Bailment
A special type of security interest, most commonly articulated in the word of finance in the shape of a pledge. A bailee has possession of but no legal title to the chattel, but this operates as good practical security, because the bailee can demand the bailor pay its bill before giving the item back.
This is why your mechanic has that smug look when you roll up in the Disco for your MOT.
To be compared with a trust relationship, where the person holding the item has legal title to it, but not possession.