Hedonic adaptation is when your baseline of happiness adapts to circumstances. I make a distinction between absolute happiness and relative happiness. Absolute happiness is a description of a level of happiness that is not compared to another standard, whereas relative happiness is the happiness relative to a standard.
Standards that relative happiness can be compared to:
Individual baseline: the level of happiness of a person in a certain moment compared to the level of happiness that person feels on average. Different individuals have different baselines, so, let's say, if Person A's baseline is 4.0 in terms of absolute happiness, and Person's B baseline is 6.0 in terms of absolute happiness, then if in a certain moment, both Person A and Person B are feeling 5.0 in terms of happiness, they are experiencing the "same" level of happiness in terms of absolute happiness, but Person A is happier than usual and Person B is not as happy as usual.
Societal baseline: the level of happiness (or even material well-being, which is not the same, but shares the same logical structure as happiness regarding "societal baseline") of an individual compared to the average level of happiness of society. In terms of material well-being, consider this example: having a smartphone in a very poor country is a luxury, whereas having a smartphone with that same cost in a very rich country is simply a standard. The distinction between luxury and standard is often made as a comparison to the societal average. A distinction not based on that, i.e. absolute luxury vs absolute standard, can be made, though the lines will necessarily be arbitrary. I've made an example of material luxury in order to illustrate the concept, but the same logic can apply to happiness, or any other quality that exists as a spectrum, e.g. the same person can be considered introverted among a group of extroverts and viceversa. The example will happiness is: the same person can be considered "serious" or even "grumpy" in a group of euphoric people, but can also be considered too "optimistic" or "cheerful" in a group of depressed, cynical or overall negative people. So, the individual baseline is the level of happiness a person has on average, and the same person can be more or less happy than average, whereas the societal baseline is the level of happiness the society has on average, and a person can be more or less happy compared to that standard.
What happens if the level of happiness rises? Some possibilities:
1: The level of happiness rises, and your expectations rise accordingly. As soon as the increased level of happiness becomes the new standard, feeling any less happy than that may make you worry of a "relapse", or something wrong happening, even though that same level of happiness was considered okay or even good before the new standard was set. Within this pattern, "sad" or "happy" are not absolutes, but relatives, and are more properly understood as "sadder than before" or "happier than before". You perceive variation of emotions across time more sharply than the emotions by themselves. To make an arbitrary example just to illustrate the concept: suppose that you like eating tomatoes. They don't give you some over-the-top pleasure, but you enjoy the taste. One day, you eat a variety of green tomatoes that you like way more than the usual red tomatoes. The first time you eat those green tomatoes, it's like heaven for your tastebuds, it brings you lots of pleasure. So, you start eating green tomatoes habitually. After a while, you still enjoy eating green tomatoes, but they just become normal. As for red tomatoes, they taste pretty bad now, you don't like them, and then you wonder how could you eat those in the past? So, you only eat green tomatoes, not the red ones. What used to be a higher pleasure has become the new normality.
2: The level of happiness rises, but your expectations do not rise accordingly. Even if you feel less happy, you still judge your level of happiness based on your previous standard. There's no hedonic adaptation here. Regarding the example made above: if you prefer green tomatoes and start eating them habitually, it's not like you like red tomatoes less than before. You like them less than green tomatoes, but your appreciation of red tomatoes remained the same as before: you enjoy them, but they don't give you over-the-top pleasure, whereas green tomatoes still feel like a bliss when you eat them.
3: Acceleration of happiness (same as 1 but with the level of happiness increasingly in accelerated manner rather than in a linear manner): your level of happiness keeps increasing constantly, creating no new standard for the level of happiness itself (as it always keeps increasing), but creating a new standard of "acceleration of happiness", which can be calculated through "points of happiness gained per month" or something like that. Of course, it cannot actually be calculated, it's just a concept, but it may (or may not) be useful as an illustration. The question is: does your mind adapt to the acceleration of happiness, or does it just adapt to the level of happiness itself?
3.1: Your mind does not adapt to a constant increase of happiness: if your happiness constantly increases, you do actually feel constantly happy, with no adaptation. It's a successful loophole against hedonic adaptation and to find long-lasting happiness! Your mind does adapt to the level of happiness itself, but if the "dose" (metaphorically speaking) keeps increasing, you'll feel constant happiness!
3.2: Your mind adapts to a constant increase of happiness as well, because your mind compares the expectation vs reality of an event. If reality surpasses the expectation, you feel happy, with a level of happiness equal to that of the difference between reality and expectation, whereas if reality is not as good as the expectation, you feel disappointed. If reality is the same as your expectation, you feel neutral. The constant increase of happiness produces an expectation, and since the expectation is met with a reality that matches that expectation, even the constant increase of happiness is something your mind adapts to, making the constant increase of happiness a failure when it comes to trying to find ways around the hedonic adaptation. However, with the logic presented in this "3.2" idea, it follows that there may be a way to defeat the hedonic adaptation: namely, to lower your expectations a lot, so that when something happens, there's a high chance it will surpass your expectations!
More may be coming soon.
Some possible ways to defeat the hedonic adaptation, and whether or not they work:
1: Seek increasingly higher pleasures, so that (in theory) hedonic adaptation never sets in. Does it work?
1.1: Yes.
1.2: Sometimes.
1.3: No.
2: In this view, it's believed that happiness is the difference between reality and expectations. The higher the difference, the higher the happiness.
2.1: Therefore, to "defeat" the hedonic adaptation, you should lower your expectations. Does it work?
2.1.1: Yes.
2.1.2: Sometimes.
2.1.3: No.