The Definition of Second Injury
Second injury is a workplace injury that occurs in addition to, or is aggravated by, a pre-existing condition or a previous injury.
Second injury is a workplace injury that occurs in addition to, or is aggravated by, a pre-existing condition or a previous injury.

A term used primarily in workers' compensation contexts to describe a new injury that combines with an earlier disability to produce a greater overall impairment than the new injury alone would cause. Some jurisdictions maintain "Second Injury Funds" to help offset employer liability in these cases.
An employee with a prior back injury from a previous job re-injures the same area in a new workplace incident, resulting in a combined disability more severe than either injury individually.
It's a state-administered fund, existing in some jurisdictions, designed to reimburse employers or insurers for a portion of workers' compensation costs when a second injury combines with a pre-existing condition.
This varies by jurisdiction and specific case facts; some workers' compensation systems apportion liability between the prior condition and the new injury, while others hold the employer responsible for the full resulting disability.
These provisions can affect workers' compensation costs and hiring decisions, since employers may be concerned about liability for aggravating a worker's pre-existing condition.