A habit (or wont, as a humorous and formal term) is a routine of behavior that is repeated regularly and tends to occur subconsciously.

A 1903 paper in the American Journal of Psychology defined a "habit, from the standpoint of psychology, [as] a more or less fixed way of thinking, willing, or feeling acquired through previous repetition of a mental experience." Habitual behavior often goes unnoticed by persons exhibiting it, because a person does not need to engage in self-analysis when undertaking routine tasks. Habits are sometimes compulsory. A 2002 daily experience study by habit researcher Wendy Wood and her colleagues found that approximately 43% of daily behaviors are performed out of habit. New behaviours can become automatic through the process of habit formation. Old habits are hard to break and new habits are hard to form because the behavioural patterns that humans repeat become imprinted in neural pathways, but it is possible to form new habits through repetition.

When behaviors are repeated in a consistent context, there is an incremental increase in the link between the context and the action. This increases the automaticity of the behavior in that context. Features of an automatic behavior are all or some of: efficiency, lack of awareness, unintentionality, and uncontrollability.

The word habit derives from the Latin words habere, which means "have, consist of," and habitus, which means "condition, or state of being." It also is derived from the French word habit (French pronunciation: [abi]), which means clothes. In the 13th century CE, the word habit first just referred to clothing. The meaning then progressed to the more common use of the word, which is "acquired mode of behavior."

In 1890, William James, a pioneering philosopher and psychologist, addressed the subject of habit in his book, The Principles of Psychology. James viewed habit as natural tendency in order to navigate life. To him, "living creatures... are bundles of habits" and those habits that have "an innate tendency are called instincts." James also explains how habits can govern our lives. He states, "Any sequence of mental action which has been frequently repeated tends to perpetuate itself; so that we find ourselves automatically prompted to think, feel, or do what we have been before accustomed to think, feel, or do, under like circumstances, without any consciously formed purpose, or anticipated of result."

Habit played an important work in the phenomenology of French philosopher Maurice Merleau-Ponty. For Merleau-Ponty, habit was a form of embodied consciousness.

Habit formation is the process by which a behavior, through regular repetition, becomes automatic or habitual. This is modeled as an increase in automaticity with the number of repetitions, up to an asymptote. This process of habit formation can be slow. Lally et al. found the average time for participants to reach the asymptote of automaticity was 66 days with a range of 18–254 days.

There are four main components to habit formation: the context cue, craving, behavioral repetition, and the reward. The context cue can be a prior action, time of day, location, or anything that triggers the habitual behavior. This could be anything that one associates with that habit, and upon which one will automatically let a habitual behavior begin. The cue leads to a craving or desire. The craving is the motivational force behind the habit. You don’t crave the habit itself; you crave the outcome you believe it will bring. The behavior is the actual habit that one exhibits, and the reward, such as a positive feeling, reinforces the "habit loop". A habit may initially be triggered by a goal, but over time that goal becomes less necessary and the habit becomes more automatic. Intermittent or uncertain rewards have been found to be particularly effective in promoting habit learning.