Attrition
Attrition has dual meanings depending on the context of its usage. Generally it means wearing or grinding down by friction. From a military standpoint it means a gradual, natural reduction in membership or personnel, as through retirement, resignation, or death. Attrition damage can be sped up by one party against another with a strategy of constant harassment, abuse, or attack or though the use of a blockade.
Attrition in HRM terms is also a Decruitment option. It's the action of not filling openings created by voluntary resignation or normal retirements.