An insider is a member of any group of people of limited number and generally restricted access. The term is used in the context of secret, privileged, hidden or otherwise esoteric information or knowledge: an insider is a "member of the gang" and as such knows things only people in the gang know.
In our complicated and information-rich world, the concept of insider knowledge is popular and pervasive, as a source of direct and useful guidance. In a given situation, an insider is contrasted with an outside expert: the expert can provide an in-depth theoretical analysis that should lead to a practical opinion, while an insider has firsthand, material knowledge. Insider information may be thought of as more accurate and valuable than expert opinion.
There are many popular cultural roles ascribed to the insider.
In criminal and social justice, whistle-blowing and leaks are seen as (often heroic) efforts of individual insiders to right wrongs by making secret information public, usually in David and Goliath situations (e.g. by revealing transgressions of governments, large corporations or other powerful organizations). When whistle-blowers are cultivated by outside forces, they are known as informants and informers.
In finance, insider trading on the stock market is widely thought of as a means of rapidly increasing wealth for the privileged few who have access to private business information. This is at times perceived as profiting unfairly at the expense of others. Some forms of insider trading are illegal.
In everyday life, insider knowledge is seen as a source of practical information that is contrary to the standard or official advice, delivered in the form of tips from insiders in a wide range of industries. Industry insiders tell outsiders what really goes on behind the scenes, so that they can take advantage of that knowledge. Related to this is personal insider access, where advantageous info can be gained through "knowing someone" ("I'll find out what really goes on from my brother the cop, or my aunt the judge, or my friend who works in accounting").
In business, tips take the form of insider knowledge used by individuals and companies, often away from the view of the public. Jobs and contracts, while openly advertised, in fact go to people based on exchanges of various inside info between key parties. Here, the insider channels can themselves be revealed to a wider audience by other insiders.
A variation of this is the insider-versus-expert authority conflict, where the uncredentaled, "non-expert" insider has a better functional knowledge of a situation, based on "how things really work", than the outside experts, whose expertise is often embodied in a set of rules. For example, there is the farmer who sees folly in new agricultural regulations created by a team of government experts without firsthand farming experience. Or the office staff that keeps a business running smoothly by unofficially ignoring and working around impractical rules devised by management consultants.
In the media, investigative reports, exposés and inside stories are popular genres that usually rely on insiders speaking out, with results ranging from the trivial to the monumentally whistle-blowing.
Insider is a fluid term: depending on the usage, it can have strongly positive, negative, or neutral connotations. It is probably safe to say that in all contexts, being an insider implies relatively great, if fleeting, personal power, and therefore engenders some form of respect, and in certain situations, fear.
Active participation is a defining factor: being a witness alone does not make a person an insider. An insider is usually one who is privy to, not simply facts and procedures, but also the day-to-day working relationships and dynamics of people in a group.
Underlying the concept of the insider is a widespread belief, and fact, that in most human social activities, there are two simultaneous, intertwined systems or processes at work: the "way things seem" and the "way they really are". In any one situation, the way things seem may or may not actually be how things work most of the time, but there is always some significant deviation, usually through the clandestine activity of small numbers of individuals within the larger group. Revealing these activities is the role of the insider.
In this way, an insider comes into existence only on or near the act of revealing what they know to an outside party. The majority of people have some area of insider knowledge, but they only become insiders when tapped for that information.
In law, a formal value is assigned to insider knowledge when it is cited to qualify a person as an expert witness. It is also recognized in intelligence and law enforcement, when insiders are deliberately created in the form of deep cover spies and undercover officers, to obtain knowledge not available through surveillance and investigation.
Two broad classes of insider are commonly recognized: those who are principal players in closely-knit groups with specific objectives (e.g. the upper management of a corporation; members of a secret society), and participants in a general area of endeavour, such as a particular industry or profession, a company, or a government department (e.g. a lawyer, an employee of XYZ Inc., a postal worker).