Adaptation-Level Phenomenon is the tendency people have to quickly adapt to a new situation, until that situation becomes the norm. Once the new situation is normal, another new experience is needed -- it constantly raises the level for what is new or exciting as each new thing becomes the norm.
For instance, you may live on a small amount of money, say, $1,000 per month. You may think "if I had more money I would always be able to pay all my bills and still buy other things." Then you get a big raise and you start making $3,000 per month. At first this would be a very exciting new experience. After a while, however, when all the new income has been allotted to pay some bill, you might again start to think, "if I had more money.... " You had a new situation, you adapted to it, and it became your normal.