In medieval times what was a small farmer who did not own land called?

Are you thinking of the Word 'serf'?

BY the way, be careful of how you use the Word 'own'. In a feudal system the only person who 'owned' the land was the king or queen. Everyone else rented it, and formed a chain of tenants. Imagine that one person (A) rents each county from the king, then one person (B) rents each village from A, and B lets each house to a different villager (C). So neither A, B nor C actually own the land, even though C lives there and A and B receive rent for C to live there.