Nostalgia is sentimental longing for the past, typically for a period or place with happy personal associations. The word nostalgia is a neoclassical compound derived from Greek, consisting of νόστος (nóstos), a Homeric word meaning "homecoming", and ἄλγος (álgos), meaning "pain"; the word 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 old days" or a "warm childhood". There is a predisposition, caused by cognitive biases such as rosy retrospection, for people to view the past more positively and the 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 provoker 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.

Nostalgia's definition has changed greatly over time. Consistent with its Greek word roots meaning "homecoming" and "pain", nostalgia was for centuries considered a potentially debilitating and sometimes fatal medical condition characterized by expressing extreme homesickness. The modern view is that nostalgia is an independent, and even positive, emotion that many people experience often. Nostalgia has been found to have important psychological functions, such as to improve mood, increase social connectedness, enhance positive self-regard, and provide existential meaning. Nostalgia can lead individuals to perceive the past more favorably than the present, a phenomenon known as the 'nostalgia effect,' which is classified as a cognitive bias. Many nostalgic reflections serve more than one function, and overall seem to benefit those who experience them. Such benefits may lead to a chronic disposition or personality trait of "nostalgia proneness." Nostalgia has also been associated with learning and memory consolidation.

Although nostalgia is often triggered by negative feelings, it results in increasing one's mood and heightening positive emotions, which can stem from feelings of warmth or coping resulting from nostalgic reflections. One way to improve mood is to effectively cope with problems that hinder one's happiness. Batcho (2013) found that nostalgia proneness positively related to successful methods of coping throughout all stages—planning and implementing strategies, and reframing the issue positively. These studies led to the conclusion that the coping strategies that are likely among nostalgia-prone people often lead to benefits during stressful times. Nostalgia can be connected to more focus on coping strategies and implementing them, thus increasing support in challenging times.

Nostalgia sometimes involves memories of people one was close to, such as family members, romantic lovers, or friends, and thus it can increase one's sense of social support and connections. Nostalgia is also triggered specifically by feelings of loneliness, but counteracts such feelings with reflections of close relationships. According to Zhou et al. (2008), lonely people often have lesser perceptions of social support. Loneliness, however, leads to nostalgia, which actually increases perceptions of social support. Thus, Zhou and colleagues (2008) concluded that nostalgia serves a restorative function for individuals regarding their social connectedness.

Nostalgia serves as a motivator for the preservation of people's cultural heritage. People endeavor to conserve buildings, landscapes, and other artifacts of historical significance out of nostalgia for past times. They are often motivated by a desire to connect to their heritage from past generations. This can manifest in living history events such as historical reenactments, which bring together people with a shared nostalgia for historical periods of past times. These events' hands-on, improvisational natures often facilitate socialization.

Nostalgia serves as a coping mechanism and helps people to feel better about themselves. Vess et al. (2012) found that the subjects who thought of nostalgic memories showed greater accessibility of positive characteristics than those who thought of exciting future experiences. Additionally, in a second study conducted, some participants were exposed to nostalgic engagement and reflection while the other group was not. The researchers looked again at self-attributes and found that the participants who were not exposed to nostalgic experiences reflected a pattern of selfish and self-centered attributes. Vess et al. (2012), however, found that this effect had weakened and become less powerful among the participants who engaged in nostalgic reflection.