The policy of giving into an aggressor to maintain peace is called?

Giving in to an aggressor would just give them the mentality of, "Well if they just let me have this, then I can just take more and more of whatever I want." This is the same thing that happened with Hitler, we gave him control of a country that he broke a treaty to obtain, making him promise that he wouldn't do it again. This didn't happen, and so started WWII.