Nostalgia is a sentimentality for the past, typically for a period or place with happy personal associations. The word nostalgia is a learned formation of a Greek compound, consisting of νόστος (nóstos), meaning "homecoming", a Homeric word, and ἄλγος (álgos), meaning "sorrow" or "despair", and was coined by a 17th-century medical student to describe the anxieties displayed by Swiss mercenaries fighting away from home. Described as a medical condition—a form of melancholy—in the Early Modern period, it became an important trope in Romanticism.

Nostalgia is associated with a longing for the past, its personalities, possibilities, and events, especially the "good ol' days" or a "warm childhood". There is a predisposition, caused by cognitive biases such as rosy retrospection, for people to view the past more favourably and future more negatively. When applied to one's beliefs about a society or institution, this is called declinism, which has been described as "a trick of the mind" and as "an emotional strategy, something comforting to snuggle up to when the present day seems intolerably bleak."

The scientific literature on nostalgia usually refers to nostalgia regarding one's personal life and has mainly studied the effects of nostalgia as induced during these studies. Emotion is a strong evoker of nostalgia due to the processing of these stimuli first passing through the amygdala, the emotional seat of the brain. These recollections of one's past are usually important events, people one cares about, and places where one has spent time. Cultural phenomena such as music, movies, television shows, and video games, as well as natural phenomena such as weather and environment can also be strong triggers of nostalgia.