Definition of Subleasing. Subleasing occurs when a tenant rents out their rental unit to another person, known as a subtenant.Clarify private housing applications and lease requirements, and discover safety tips for finding legitimate rentals. A sublease, or a sublease agreement, is when a landlord and tenant agree to rent out the unit to another resident not originally on the lease agreement. Subleasing occurs when a tenant rents out the property to someone else but remains responsible for the lease terms with the landlord. Mortgage, security interest, pledge, gift, or any other voluntary transaction creating an interest in goods. Illinois Sublease Agreement is a document between a tenant and a subtenant whom agrees to rent the same property. Roommate A roommate is a person who lives in the same rental property. They would also be a signed party on the original lease agreement. Subletting is renting out an apartment, or any property, that has your name on the lease.