Kinetic Energy
The energy the bullet carries in flight, expressed in foot-pounds in US convention. A function of mass and the square of velocity.
Velocity matters more than weight because energy scales with velocity squared. Doubling the weight doubles the energy; doubling the velocity quadruples it.
For precision shooting this is mostly a hunting metric, terminal performance on game. For target work at distance, retained velocity (specifically staying supersonic) matters more than kinetic energy in the abstract.