The term "ultimate sacrifice" refers to an act of selflessness and bravery, where a person gives up their life or something they deeply cherish in order to protect others or serve a greater cause. It is a concept that represents the highest level of dedication and commitment, often associated with heroism and honor. The phrase can be used to describe any act of sacrifice, but it usually refers specifically to military service members who make the ultimate sacrifice by laying down their lives in defense of their country or its citizens.