a lease of part or all of a property to a subtenant by a tenant who is still on an original lease with the landlord, or (informal) property that is available under such an agreement;
e.g. He is looking for a sublet to live in for the summer.
(verb)
to lease part or all of a property to a subtenant while remaining on a lease with the landlord, describing a tenant's actions;
e.g. She is willing to sublet the room to anyone who needs a place to stay.