Penetration
How much armor a shell can defeat, in millimetres, rolled plus or minus 25 percent on every shot and compared against the effective thickness it hits.
Also known as pen, armor penetration, piercing power.
A shell penetrates when its rolled penetration beats the effective armor at the point of impact. Effective thickness is not what the specification sheet says: the angle of the plate, the shell's normalization and the distance travelled all change it before the comparison happens.
The plus or minus 25 percent roll is why marginal shots are a lottery. A 220 millimetre gun against 215 effective armor penetrates most of the time and bounces often enough to matter, and the same roll is why a shot that should never get through occasionally does.
Armor-piercing and APCR shells lose penetration with distance, which is why the value is quoted both at the muzzle and at 500 metres. HEAT and high explosive do not lose any.