Vesting
Vesting is the process of earning full ownership of employer-contributed money or stock over time. Your own contributions are always 100% yours, but employer matches often vest gradually — for example 20% per year over five years. Leaving a job before you are fully vested forfeits the unvested portion.
Example
If $10,000 of employer match is on a 4-year vesting schedule and you leave after 2 years, you keep only $5,000.