A habit (or wont) is a routine of behavior that is repeated regularly and tends to occur subconsciously. In the American Journal of Psychology (1903) it is defined in this way: "A habit, from the standpoint of psychology, is a more or less fixed way of thinking, willing, or feeling acquired through previous repetition of a mental experience." Habitual behavior often goes unnoticed in persons exhibiting it, because a person does not need to engage in self-analysis when undertaking routine tasks. Habits are sometimes compulsory. The process by which new behaviours become automatic is habit formation. Old habits are hard to break and new habits are hard to form because the behavioural patterns we repeat are imprinted in our neural pathways, but it is possible to form new habits through repetition.
As 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, uncontrollability.
In fact, the habit formation is a slow process. Lally et al. (2010) found the average time for participants to reach the asymptote of automaticity was 66 days with a range of 18–254 days. Automaticity increases along an asymptotic curve, which is unique to each individual. 
Habits form in three parts: there is the cue, the behavior, and the reward. The cue would be the thing that causes your habit to come about; the trigger to your habit. This could be anything that your mind associates with that habit and you will automatically let a habit come to the surface. The behavior is the actual habit that you are exhibiting and the reward is your brain liking it therefor continuing 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 innate.
Habits and goals
The habit–goal interface is constrained by the particular manner in which habits are learned and represented in memory. Specifically, the associative learning underlying habits is characterized by the slow, incremental accrual of information over time in procedural memory. Habits can either benefit or hurt the goals a person sets for themselves.
Goals guide habits by providing the initial outcome-oriented motivation for response repetition. In this sense, habits are often a trace of past goal pursuit. Although, when a habit forces one action, but a conscious goal pushes for another action, an op-positional context occurs. When the habit prevails over the conscious goal a capture error has taken place.
Behavior prediction is also derived from goals. Behavior prediction is to acknowledge a habit will form, but in order to form that habit, a goal must have been initially present. The influence of goals on habits is what makes a habit different from other automatic processes in the mind.
Habits as described by animal behavior experiments
The following is from a Scientific American MIND Guest Blog post called Should Habits or Goals Direct Your Life? It Depends.
"A series of elegant experiments  conducted by Anthony Dickinson and colleagues in the early 1980s at the University of Cambridge in England clearly exposes the behavioral differences between goal-directed and habitual processes. Basically, in the training phase, a rat was trained to press a lever in order to receive some food. Then, in a second phase, the rat was placed in a different cage without a lever and was given the food, but it was made ill whenever it ate the food. This caused the rat to “devalue” the food, because it associated the food with being ill, without directly associating the action of pressing the lever with being ill. Finally, in the test phase, the rat was placed in the original cage with the lever. (To prevent additional learning, no food was delivered in the test phase.) Rats that had undergone an extensive training phase continued to press the lever in the test phase even though the food was devalued; their behavior was called habitual. Rats that had undergone a moderate training phase did not, and their behavior was called goal-directed. ... Goal-directed behavior is explained by the rat using an explicit prediction of the consequence, or outcome, of an action to select that action. If the rat wants the food, it presses the lever, because it predicts that pressing the lever will deliver the food. If the food has been devalued, the rat will not press the lever. Habitual behavior is explained by a strong association between an action and the situation from which the action was executed. The rat presses the lever when it sees the lever, not because of the predicted outcome."
Habits and nervousness
There are a number of habits possessed by individuals that can be classified as nervous habits. These include nail-biting, stammering, sniffling, and banging the head. They are known as symptoms of an emotional state and are generally based upon conditions of anxiety, insecurity, inferiority and tension. These habits are often formed at a young age and may be because of a need for attention. When trying to overcome a nervous habit it is important to resolve the cause of the nervous feeling rather than the symptom which is a habit itself 
A bad habit is an undesirable behavior pattern. Common examples include: procrastination, fidgeting, overspending, nail-biting. The sooner one recognizes these bad habits, the easier it is to fix them.
Will and intention
A key factor in distinguishing a bad habit from an addiction or mental disease is the element of willpower. If a person still seems to have control over the behavior then it is just a habit. Good intentions are able to override the negative effect of bad habits but their effect seems to be independent and additive—the bad habits remain but are subdued rather than cancelled.
Eliminating bad habits
There are many techniques for removing bad habits once they have become established. One example is withdrawal of reinforcers—identifying and removing the factors which trigger the habit and encourage its persistence. The basal ganglia appears to remember the context that triggers a habit, meaning they can be revived if triggers reappear. Furthermore, it's better if you recognize your bad habits and eliminate them as soon as possible. As you get older, it becomes more difficult to remove them because the many repetitions have helped with the building of the habit, and each repetition has unfortunately left its mark.
Use in litigation
Habit evidence is a term used in the law of evidence in the United States to describe any evidence submitted for the purpose of proving that a person acted in a particular way on a particular occasion based on that person's tendency to reflexively respond to a particular situation in a particular way. Habit evidence differs from character evidence, which seeks to show that a person behaved in a particular way on a particular occasion based on things like that person's prior bad acts or reputation in the community, and which is generally inadmissible.
- Butler, Gillian; Hope, Tony. Managing Your Mind: The mental fitness guide. Oxford Paperbacks, 1995
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- Arthur Payne,(n.d) The psychology of nervous habits
- James Rowland Angell and Addison W. Moore. (1896) "Studies from the Psychological Laboratory of the University of Chicago: 1. Reaction-Time: A Study in Attention and Habit." Psychological Review 3, 245–258.