A security deposit is a payment required by a landlord from a tenant to cover the expenses of any repairs of damages to the premises greater than normal "wear and tear." Everyday usage is normal wear and tear, but excess damage is a debated definition. A security deposit is not the same as rent. It is money that actually belongs to the tenant but is held by the landlord for tenant-caused damages and sometimes past-due rent. Without the agreement of the landlord, a security deposit may not legally be used as the last month's rent.
Laws vary by state, but some states place a limit on the amount of a security deposit that a landlord may charge. Some states also regulate where residential security deposits must be kept and when interest payments on the security deposits must be made to the tenant. State laws also define the time period after the tenant vacates within which the deposit must be returned to the tenant.