As stated earlier, Post Mint Damage. It's when something happens to the coin after getting into circulation.
Simple questions like this can be easily answered on www.google.com or use the search function on this forum...I understand that you are only 9 though I want to pave the way to help you get in the right direction now while you are young then constantly relying on people. Here is what PMD is: http://coins.thefuntimesguide.com/2010/08/pmd.php Here are some examples of PMD Coins: http://www.cointalk.com/t168596/
Come on shoe....get with the proper linking for these type of questions! LOL http://lmgtfy.com/?q=pmd+coin
But to me this brings up the question of when does damage become post Mint? Is ANY "normal" damage that occurs within the Mint facility not post-Mint? What about damage that occurs during the transfer of coins from the Mint to a Federal Reserve bank? Or does a coin truly have to leave Gov't control and be placed in circulation before it can be post-Mint?
Hmmmmmmm, yes. Another term to define. What is damage? Is a lamination damage? Double/Multiple strikes? All the other weird things that can happen during the striking process? Coin striking coin after leaving the striking chamber and sliding down into the collecting bin?
Interesting. So once a coin slides down into the bin, anything that happens there is post-Mint. Those dings caused by a reeded edge to a coin already in the bin is post-Mint!? I'm not quite in agreement.
You got it. You said you are not quite in agreement but the nice thing is, we generally don't have to agree and we can collect stuff that others would never look twice at no margret how it was made. They just won't command a premium except maybe a small amount amongst others that share an interest in those types. I just read somewhere this week about a few possible RPMs that could also be PMD from another coin strike. The experts can't be 100% sure of the origin of the mark, but some people collect them anyway. If only my bogged down brain could remember which dates and what site I saw it on...
PMD == Pre Menstrual Disorder Coin collectors are advised to not spend large sums of money on coins during their wives/girlfriends PMD. Unless said expenditures can be negated by large sums of Midol.