Most people would call it a counterstamp, I just call it damage.
There are counterstamps, contermarks, and damage or mutilation.
A counterstamp is something punched into the coin to advertise a person, place, event,or thing. The punching has a definite meaning or purpose.
A countermark is a stamping into a coin or token, usually by a government, to establish its value. Typically through revaluing older coins or by making coins of another country to make them current circulating tender in that country. Businesses will also sometimes do this stamping foreign coins, obsolete tokens, or tokens of other establishments in order to mark them as being acceptable in their place of business.
Then there are just random punches of letters, numbers, or symbols that have no apparent definite meaning. This is just damage. From time to time research will detemine that one of these apparently randon punches does have a specific meaning which will turn it into a counterstamped piece, but that very seldom happens and the vast majority of coins with random things punched into them is just damage.
The problem is that as the collecting of counterstamps has become more "acceptable" dealers are starting to put silly prices on any coin that has a random letter or number punched into it and call it a counterstamp, or even worse an "Unlisted Counterstamp" and slapping a high price on it. (Take a worn coin worth a dollar, stamp a letter into it, call it an unlisted counterstamp, and slap a $25 price tag on it.)