A "work anniversary" means the date when someone's employment began, marking the number of years they have been working at a particular company or in a specific job. Full definition
If you leave before your third work anniversary, you only get to keep 25 percent of the amount your employer contributed to your 401 (k) account. (creditsesame.com)
If you wait until after your fifth work anniversary, you get to keep it all. (creditsesame.com)
You keep all of your contributions no matter how long you worked there, but your employer match may be contingent on reaching a specific work anniversary. (thecollegeinvestor.com)