A subtenant is someone who rents a place, or part of one, from a tenant rather than from the owner. Picture three people in a line: the owner, the tenant who rents from the owner, and the subtenant who rents on from that tenant. The subtenant pays rent to the tenant, not to the landlord, and usually has no agreement with the owner at all — which is why a subtenant's position is weaker. If the tenant loses the flat, the subtenant normally has to go too. The word is legal and formal; in normal speech people say 'I'm subletting from someone' or 'I rent a room off my friend who rents the flat'. It is also used as a verb occasionally, meaning to hold or let a place as a subtenant, but that is rare and the noun is what you will actually see.