Muzzle energy
The kinetic energy of a projectile as it leaves the muzzle: one half the mass times the velocity squared.
A calculation, not a measurement. Every energy figure on this site is computed from a stored bullet mass and a stored velocity, which is why an energy is never recorded as a fact in its own right — storing it would create a second copy of a derived number, free to drift away from the inputs it came from.
Because velocity is squared, energy is dominated by it: doubling the mass doubles the energy, doubling the velocity quadruples it. That is the whole argument for small, fast cartridges.
What muzzle energy is not is a measure of effect. This site publishes energy, momentum and retained velocity because they are physics, and publishes no damage, lethality or stopping-power figure of any kind, because those are not.