Full Definition
A co-signer (also called a guarantor) is someone who signs the lease alongside the primary renter and agrees to pay the rent if the primary renter defaults. The co-signer doesn't live in the apartment — they're just providing the financial backstop.
Most apartment communities require the co-signer to: have credit at 700+ (sometimes 650+), have income at 4-5x the monthly rent, be a U.S. citizen or permanent resident, and provide the same documentation as the primary applicant.
Common co-signer arrangements: parent for an adult child's first apartment, family member for someone with ITIN or recent immigration, friend in business arrangements (less common, higher risk for the friend).