Identity is the set of qualities, beliefs, personality traits, appearance, or expressions that characterize a person or a group.

Identity emerges during childhood as children start to comprehend their self-concept, and it remains a consistent aspect throughout different stages of life. Identity is shaped by social and cultural factors and how others perceive and acknowledge one's characteristics. The etymology of the term "identity" from the Latin noun identitas emphasizes an individual's "sameness with others". Identity encompasses various aspects such as occupational, religious, national, ethnic or racial, gender, educational, generational, and political identities, among others.

Identity serves multiple functions, acting as a "self-regulatory structure" that provides meaning, direction, and a sense of self-control. It fosters internal harmony and serves as a behavioral compass, enabling individuals to orient themselves towards the future and establish long-term goals. As an active process, it profoundly influences an individual's capacity to adapt to life events and achieve a state of well-being. However, identity originates from traits or attributes that individuals may have little or no control over, such as their family background or ethnicity.

In sociology, emphasis is placed by sociologists on collective identity, in which an individual's identity is strongly associated with role-behavior or the collection of group memberships that define them. According to Peter Burke, "Identities tell us who we are and they announce to others who we are." Identities subsequently guide behavior, leading "fathers" to behave like "fathers" and "nurses" to act like "nurses".

In psychology, the term "identity" is most commonly used to describe personal identity, or the distinctive qualities or traits that make an individual unique. Identities are strongly associated with self-concept, self-image (one's mental model of oneself), self-esteem, and individuality.^{[page needed]} Individuals' identities are situated, but also contextual, situationally adaptive and changing. Despite their fluid character, identities often feel as if they are stable ubiquitous categories defining an individual, because of their grounding in the sense of personal identity (the sense of being a continuous and persistent self).

Mark Mazower noted in 1998: "At some point in the 1970s this term ["identity"] was borrowed from social psychology and applied with abandon to societies, nations and groups."

Erik Erikson (1902–94) became one of the earliest psychologists to take an explicit interest in identity. An essential feature of Erikson's theory of psychosocial development was the idea of the ego identity (often referred to as the self), which is described as an individual's personal sense of continuity. He suggested that people can attain this feeling throughout their lives as they develop and is meant to be an ongoing process. The ego-identity consists of two main features: one's personal characteristics and development, and the culmination of social and cultural factors and roles that impact one's identity. In Erikson's theory, he describes eight distinct stages across the lifespan that are each characterized by a conflict between the inner, personal world and the outer, social world of an individual. Erikson identified the conflict of identity as occurring primarily during adolescence and described potential outcomes that depend on how one deals with this conflict. Those who do not manage a resynthesis of childhood identifications are seen as being in a state of 'identity diffusion' whereas those who retain their given identities unquestioned have 'foreclosed' identities. On some readings of Erikson, the development of a strong ego identity, along with the proper integration into a stable society and culture, lead to a stronger sense of identity in general. Accordingly, a deficiency in either of these factors may increase the chance of an identity crisis or confusion.

The "Neo-Eriksonian" identity status paradigm emerged in 1966, driven largely by the work of James Marcia. This model focuses on the concepts of exploration and commitment. The central idea is that an individual's sense of identity is determined in large part by the degrees to which a person has made certain explorations and the extent to which they have commitments to those explorations or a particular identity. A person may display either relative weakness or strength in terms of both exploration and commitments. When assigned categories, there were four possible results: identity diffusion, identity foreclosure, identity moratorium, and identity achievement. Diffusion is when a person avoids or refuses both exploration and making a commitment. Foreclosure occurs when a person does make a commitment to a particular identity but neglected to explore other options. Identity moratorium is when a person avoids or postpones making a commitment but is still actively exploring their options and different identities. Lastly, identity achievement is when a person has both explored many possibilities and has committed to their identity.