Flow in positive psychology, also known colloquially as being in the zone or focused, is the mental state in which a person performing some activity is fully immersed in a feeling of energized focus, full involvement, and enjoyment in the process of the activity. In essence, flow is characterized by the complete absorption in what one does, and a resulting transformation in one's sense of time. Flow is the melting together of action and consciousness; the state of finding a balance between a skill and how challenging that task is. It requires a high level of concentration. Flow is used as a coping skill for stress and anxiety when productively pursuing a form of leisure that matches one's skill set.

First presented in the 1975 book Beyond Boredom and Anxiety by the Hungarian-American psychologist Mihály Csíkszentmihályi, the concept has been widely referred to across a variety of fields (and is particularly well recognized in occupational therapy).

The flow state shares many characteristics with hyperfocus. However, hyperfocus is not always described in a positive light. Some examples include spending "too much" time playing video games or becoming pleasurably absorbed by one aspect of an assignment or task to the detriment of the overall assignment. In some cases, hyperfocus can "capture" a person, perhaps causing them to appear unfocused or to start several unfinished projects. Hyperfocus is often mentioned "in the context of autism, schizophrenia, and attention deficit hyperactivity disorder – conditions that have consequences on attentional abilities."

Flow is an individual experience and the idea behind flow originated from the sport psychology theory about an Individual Zone of Optimal Functioning. The individuality of the concept of flow suggests that each person has their subjective area of flow, where they would function best given the situation. One is most likely to experience flow at moderate levels of psychological arousal, as one is unlikely to be overwhelmed, but not understimulated to the point of boredom.

Flow is so named because, during Csíkszentmihályi's 1975 interviews, several people described their "flow" experiences using the metaphor of a water current carrying them along:

We have called this state the flow experience, because this is the term many of the people we interviewed had used in their descriptions of how it felt to be in top form: "It was like floating," "I was carried on by the flow."

— Csikszentmihályi, Flow (1990)

Mihaly Csikszentmihályi and others began researching flow after he became fascinated by artists who would essentially get lost in their work. Artists, especially painters, got so immersed in their work that they would disregard their need for food, water and even sleep. The theory of flow came about when Csikszentmihályi tried to understand the phenomenon experienced by these artists. Flow research became prevalent in the 1980s and 1990s, with Csikszentmihályi and his colleagues in Italy still at the forefront. Researchers grew interested in optimal experiences and emphasizing positive experiences, especially in places such as schools and the business world. They also began studying the theory of flow at this time.