Definition
A nine-digit number from the Social Security Administration used to track earnings for benefits and to identify people for taxes.
What it means
The Social Security Administration issues SSNs to U.S. citizens and to noncitizens who are authorized to work, and in limited cases to others with a valid non-work reason. Your SSN records your earnings so you can earn credits for Social Security and Medicare, and it serves as your taxpayer identification number. Employers need it for payroll and Form W-2, and banks and lenders use it to verify identity and check credit. Some cards are marked to show work restrictions. Many noncitizens can apply for an SSN when they apply for a work permit or immigrant visa. Applying is free. Protecting the number matters because thieves use it for identity theft; SSA does not suspend numbers or demand payment by phone.
Example
A new permanent resident who checked the SSN box on the immigrant visa application usually receives the card by mail after arriving, without a separate visit to an SSA office.
Illustrative numbers only. Your situation will differ.