DEFINITION of 'Retention Bonus'. A payment or reward outside of an employee's regular salary that is offered as an incentive to keep a key employee on the job during a particularly crucial business cycle, like a merger or acquisition, or during a crucial production period.REAKING DOWN 'Retention Bonus'. With the corporate landscape changing almost daily and a liquid labor market allowing workers to move from job to job more easily, retention bonuses have provided a great way for companies to keep valuable employees.