Anonymity is derived from the Greek word ????????, meaning "without a name" or "namelessness". In colloquial use, the term 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".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 that opened the safe; the perpetrator remains anonymous. In particular, each of the elements in has a 1/3 chance of being the perpetrator
Source:
http://en.wikipedia.org/wiki/Anonymous