The psychological tendency for people to return to a relatively stable baseline level of happiness after major positive or negative life events.
The length of time adaptation requires varies by person, context, and subject: research and popular summaries generally put the peak of the initial happiness boost at days to a few months, with a substantial return toward baseline often occurring within 3–12 months for many positive events, and fuller adaptation commonly within 1–3 years.
Psychologists recommend focusing more on meaningful shared activities, deep personal relationships, and other ongoing intentional practices (mindfulness, gratitude, etc) that resist rapid adaptation. Professionals make this recommendation because one-time external upgrades (new car, house, raise, gadget, status symbols) are typically static states. Once the novelty fades, your brain treats them as the new baseline and the emotional boost largely disappears.
By contrast, certain activities, relationships, and practices function more like ongoing processes—they keep generating variety, novelty, engagement, meaning, or social connection, which slows or partially resists adaptation.