Power Creep is when new items are introduced to a game which are so much stronger than the previous set, that people who have spent lots of time/money getting the older items are now outmatched by those who have the newer items. Eventually, instead of balancing the power, you just continue to make newer items that overpower the last, instead of incorporating the older, now weaker, set.
For example: the game galaxyonline 2 in there were only use to be skill cards in the game, then they introduced a card to completely overpower all the skill cards, then they introduce titan divines, eventually those who started when there were only skill cards and spent a lot of money had now quit the game
