Anonymity is derived from the Greek word ανωνυμία, meaning "without a name" or "namelessness". In colloquial use, anonymous typically refers to a person, and often means that the personal identity, or personally identifiable information of that person is not known. More strictly, and in reference to an arbitrary element (e.g. a human, an object, a computer), within a well-defined set (called the "anonymity set"), "anonymity" of that element refers to the property of that element of not being identifiable within this set. If it is not identifiable, then the element is said to be "anonymous". The term "anonymous message" typically refers to message (which is, for example, transmitted over some form of a network) that does not carry any information about its sender and its intended recipient. It is therefore unclear if multiple such messages have been sent by the same sender or if they have the same intended recipient. Sometimes it is desired that a person can establish a long-term relationship (such as a reputation) with some other entity, without his/her personal identity being disclosed to that entity. In this case, it may be useful for the person to establish a unique identifier, called a pseudonym, with the other entity. Examples of pseudonyms are nicknames, credit card numbers, student numbers, bank account numbers, and IP addresses. A pseudonym enables the other entity to link different messages from the same person and, thereby, the maintenance of a long-term relationship. Although typically pseudonyms do not contain personally identifying information, communication that is based on pseudonyms is often not classified as "anonymous", but as "pseudonymous" instead. Indeed, in some contexts, anonymity and pseudonymity are separate concepts. However, in other contexts what matters is that both anonymity and pseudonymity are concepts that are, among other things, concerned with hiding a person's legal identity. In such contexts people may not distinguish between anonymity and pseudonymity. The problem of determining whether or not the identity of a communication partner is the same as one previously encountered is the problem of authentication. An example: Suppose that only Alice, Bob, and Carol have the keys to a bank safe and that, one day, the contents of the safe are missing (without the lock being violated). Without any additional information, we do not know for sure whether it was Alice, Bob or Carol who opened the safe; the perpetrator remains anonymous. In particular, each of the elements in {Alice, Bob, Carol} has a 1/3 chance of being the perpetrator. However, as long as none of them has been identified as being the perpetrator with 100% certainty, we can say that the perpetrator remains anonymous. Anonymity is not an absolute. That is, the degree of anonymity one enjoys may vary. In the above example, if Carol has an ironclad alibi at the time of the perpetration, then we may deduce that it must have been either Alice or Bob who opened the safe. That is, the probability of the elements {Alice, Bob, Carol} of being the perpetrator is now 1/2, 1/2, and 0 respectively. This clearly amounts to a reduction of the perpetrator's anonymity (i.e. although the perpetrator still remains anonymous, it is now more likely than before that (s)he is either Alice or Bob). Anonymity is a result of not having identifying characteristics (such as a name or description of physical appearance) disclosed. This can occur from a lack of interest in learning the nature of such characteristics, or through intentional efforts to hide these characteristics. An example of the former would include a brief encounter with a stranger, when learning the other person's name is not deemed necessary. An example of the latter would include someone hiding behind clothing that covers identifying features like hair color, scars, or tattoos, in order to avoid identification. In some cases, anonymity is reached unintentionally, as is often the case with victims of crimes or war battles, when a body is discovered in such a state that the physical features used to identify someone are no longer present. Anonymity is not always found in such morbid situations, however. As an example, a winner of a lottery jackpot is anonymous (one of however many play the lottery) until that person turns in the winning lottery ticket. There are many reasons why a person might choose to obscure their identity and become anonymous. Several of these reasons are legal and legitimate - many acts of charity are performed anonymously, as benefactors do not wish, for whatever reason, to be acknowledged for their action. Someone who feels threatened by someone else might attempt to hide from the threat behind various means of anonymity, a witness to a crime can seek to avoid retribution, for example, by anonymously calling a crime tipline. There are also many illegal reasons to hide behind anonymity. Criminals typically try to keep themselves anonymous either to conceal the fact that a crime has been committed, or to avoid capture. Anonymity may also be created through a gradual eroding of ownership information. This particularly applies to quotations deemed anonymous. For example, the quote, "Ignorance is Bliss" originally had a known author, but, over time, information on author's identity was obscured and has disappeared. One way this authorship is obscured is through the evolution of the phrase by modification or addition to the original, which has become particularly prolific in modern times. For example, "Ignorance is Bliss" is only in the form it is in now through a long train of evolution. In fact the phrase has most recently received a modern modification in some areas resulting in a new (and much more cynical) anonymous phrase "Ignorance is Bliss...and I know everything." Anonymity may reduce the accountability one perceives to have for their actions, and removes the impact these actions might otherwise have on their reputation. This can have dramatic effects, both useful and harmful. In conversational settings, anonymity may allow people to reveal personal history and feelings without fear of later embarrassment. Electronic conversational media can provide physical isolation, in addition to anonymity. This prevents physical retaliation for remarks, and prevents negative or taboo behavior or discussion from tarnishing the reputation of the speaker. This can be beneficial when discussing very private matters, or taboo subjects or expressing views or revealing facts which may put someone in physical, financial, or legal danger (such as illegal activity, or unpopular or outlawed political views). With few perceived negative consequences, anonymous or semi-anonymous forums often provide a soapbox for disruptive conversational behavior. The term Internet troll is sometimes used to refer to those who do this online. Relative anonymity is often enjoyed in large crowds. Different people have different psychological and philosophical reactions to this development, especially as a modern phenomenon. This anonymity is an important factor in crowd psychology. Anonymity is also often referred to as simply Anon. Anonymous commercial transactions can protect the privacy of consumers. Some consumers prefer to use cash when buying everyday goods (like groceries or tools), to prevent sellers from aggregating information or soliciting them in the future. Credit cards are linked to a person's name, and can be used to discover other information, such as postal address, phone number, etc. The ecash system was developed to allow secure anonymous transactions. When purchasing taboo goods and services, anonymity makes many potential consumers more comfortable with or more willing to engage in the transaction. Many loyalty programs use cards which personally identify the consumer engaging in each transaction (possibly for later solicitation, or for redemption or security purposes), or which act as a numerical pseudonym, for use in data mining. Anonymity can also be used as a protection against legal prosecution. For example, when committing a robbery, many criminals will obscure their faces to avoid identification. In organized crime, groups of criminals may collaborate on a certain project without revealing to each other their names or other personally identifiable information. The movie The Thomas Crown Affair depicted a fictional collaboration by people who had never previously met and did not know who had recruited them. The anonymous purchase of a gun or knife to be used in a crime helps prevent linking an abandoned weapon to the identity of the perpetrator. There are two aspects, one, giving to a large charitable organization obscures the beneficiary of a donation from the benefactor, the other is giving anonymously to obscure the benefactor both from the beneficiary and from everyone else. There are many reasons this is done. Anonymous charity has long been a widespread and durable moral precept of many ethical and religious systems, as well as being in practice a widespread human activity. A benefactor may not wish to establish any relationship with the beneficiary, particularly if the beneficiary is perceived as being unsavory. Benefactors may not wish to identify themselves as capable of giving. A benefactor may wish to improve the world, as long as no one knows who did it, out of modesty, wishing to avoid publicity.[1] Attempts at anonymity are not always met with support from society. There is a trend in society to mistrust someone who makes an effort to maintain their anonymity. This is often summed up in the statement, "You wouldn't want to stay anonymous unless you had something to hide." The implication is that there is no legitimate reason to obscure one's identity from the world as a whole. Anonymity sometimes clashes with the policies and procedures of governments or private organizations. In the United States, disclosure of identity is required to be able to vote. In airports in most countries, passengers are not allowed to board flights unless they have identified themselves to some sort of airline or transportation security personnel, typically in the form of the presentation of an identification card.