It's what happens to older guns.
When you're young and just starting out buying new guns the end of the barrel is steady, but when the gun gets older it tends to shake around more, so it's called "end shake" or "endshake". Some folks say it's just me getting older but that can't possibly be, it has to be the gun....
OR
It's the front to back movement of the cylinder. Some revolvers will display a tiny bit of movement right out of the box which isn't detrimental, but a lot of heavy recoil can increase it. My 6" GP100 .357 which is about 10 years old shows just a teensy bit of movement but it always has, I'd guess no more than a few ten thousandths, whereas a 67 year old S&W K38 .38 Spl I bought recently has absolutely zero movement at all.
