Since the Thirteenth Amendment draws a distinction between the two, what was the legal definition of the terms?
IANACL but I believe the technical difference is that a “slave” in this sense refers to “chattel” slavery, wherein you as an individual are a piece of property that can be bought, sold, given, traded by the master as would a head of cattle or a piece of furniture. While “servitude” referred to such things as serfdom or indentured servitude where your master or lord cannot buy and sell YOU but you are bound to the land or a writ of indenture and while “you” as a human are not owned, you have no legal rights over your own labor (e.g. to quit, strike, demand better terms, change jobs, move elsewhere, seek a better master, limit what kind of work do you do, file complaints, etc.) and whatever fruit of that labor you keep is dependent on the master/lord’s largesse.
JRDelirious has it more or less. “Servitude” doesn’t imply ownership, just forced labor, often contractually. For example, a court can’t force an NBA player to play ball for a basketball team even though he’s signed a contract; to do so would be involuntary servitude that would violate the Thirteenth Amendment. What they can do, though, is issue an injunction against his playing for any other team.
Just to add a note to what was posted:
John Reich, the man who designed/engraved our(US) half dollar which was used from 1807-1839, came over from Germany in the early 1800’s as an indentured servant. He had his passage paid, but he had to work for his sponsor for a specified time. But he was free to do as he pleased after that time.
Findlaw on Situations in Which the Amendment Is Inapplicable including for example, a military draft.