Also called: 401(k) match
Money your employer adds to your 401(k) based on what you contribute. It does not count against your own deferral limit, and it may vest, meaning become yours, over several years.
Matching contributions are traditionally pre-tax even when your own contributions are Roth, so they are subject to required minimum distributions later. Cliff vesting grants full ownership at one date; graded vesting grants it in yearly steps.