Confidence has a common meaning of a certainty about handling something, such as work, family, social events, or relationships. Some have ascribed confidence as a state of being certain either that a hypothesis or prediction is correct or that a chosen course of action is the best or most effective. Self-confidence is having confidence in one's self. Arrogance or hubris in this comparison is having unmerited confidence--believing something or someone is capable or correct when they are not. Overconfidence or presumptuousness is excessive belief in someone (or something) succeeding, without any regard for failure. Confidence can be a self-fulfilling prophecy as those without it may fail or not try because they lack it and those with it may succeed because they have it rather than because of an innate ability. Taken to an extreme, over-confidence can cause problems as evidenced by the famous author Matthew Syed and mentioned here in this reference in regard to sport. 
Motivation theories have suggested that successful performance hinges on both skill and will. Yet, even a motivated and skilled individual can fail to perform if he or she does not have a personal certainty belief that s/he can handle what it takes or what needs to be done.Stajkovic (2006) argues that all three - skill, will, and confidence - are needed for action to unfold. Stajkovic (2006) holds that confidence resides within individuals, and is not immediately accessible to the outside world.
Self-confidence does not necessarily imply "self-belief" or a belief in one's ability to succeed. For instance, one may be inept at a particular sport or activity, but remain "confident" in one's demeanor, simply because one does not place a great deal of emphasis on the outcome of the activity. When one does not dwell on negative consequences one can be more "self-confident" because one is worrying far less about failure or the disapproval of others following potential failure. One is then more likely to focus on the actual situation which means that enjoyment and success in that situation is also more probable. Belief in one's abilities to perform an activity comes through successful experience and may add to, or consolidate, a general sense of self-confidence.[original research?] Studies have also found a link between high levels of confidence and wages. Seemingly, those who self-report they were confident earlier in schooling earned better wages and were promoted more quickly over the life course.
In certain fields of medical practice patients experience lack of self-confidence during the recovery period. This is commonly referred to as DSF or "defectum sui fiducia" from the latin etymology of lack of self-confidence. For example, this can be the case after stroke whereby the patient refrains from using the weaker lower limb due to fear of it not being strong to hold their weight whilst standing or walking.
People may have confidence in other people or forces beyond their control. For instance, one might have confidence in the police to protect them, or might have confidence that a sports team will win a game. Faith and trust are synonyms of confidence when used in this sense.
It is suggested that the confidence bias can be explained by a noisy conversion of objective evidence (observation) into subjective estimates (judgment), whereas noise is defined as the mixing of memories during the storing (observing/learning) and retrieval process (remembering/judgment). The information-theoretic logic behind this explanation is very similar to the mechanism that can also lead to the conservatism bias, and holds that we mix true and false evidence during storage and retrieval of evidence to and from our memories. The confidence bias results because as judges we "look inside our own memory" (evaluate our confidence) and find evidence that is more extreme than when we retrieve evidence for our judgements (which are conservative due to mixing of extreme values during retrieval). This explanation is very simple and straightforward, but nevertheless sufficient mechanism to generate both, overconfidence (in situations where judges are very sure) and underconfidence (in cases when judges openly state to lack the required knowledge).