A hollow point is an expanding bullet that has a pit or hollowed out shape in its tip, generally intended to cause the bullet to thin upon entering a target in order to decrease penetration and disrupt more tissue

Tissue (biology)

Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...

 as it travels through the target. It is also used for controlled penetration, where over-penetration could cause collateral damage

Collateral damage

Collateral damage is damage to people or property that is unintended or incidental to the intended outcome. The phrase is prevalently used as an euphemism for civilian casualties of a military action.-Etymology:...

 (such as on an aircraft). In essence, the hollow point bullet has two interrelated purposes: to increase its size once within the target, thus maximizing tissue damage and blood loss or shock, and by remaining in the target to expend all of its kinetic energy in the target, some of which will be lost if the bullet continues through the target.