A very old type of lawsuit to recover property (or the value of property) that was wrongly taken away. The person who takes your property is said to have "converted" it to his own use; sometimes, the action of trover is referred to as "trover and conversion." Trover is an archaic Anglo-French word meaning "to find" -- in modern French it's trouver. In English common law, the plaintiff in a trover case was required to plead that he lost his property and that the defendant had happened upon it.
You can still bring a case for trover in state court. Or at least, there are legal form companies that will sell you pre-printed trover complaints.