is derived from the Greek word ἀνωνυμία, anonymia, meaning "without a name" or "namelessness". In colloquial use, anonymity typically refers to the state of an individual's personal identity, or personally identifiable information, being publicly unknown.
There are various situations in which a person might choose to withhold their identity. Acts of charity
have been performed anonymously when benefactors do not wish to be
acknowledged. A person who feels threatened might attempt to mitigate
that threat through anonymity. A witness to a crime might seek to avoid
retribution, for example, by anonymously calling a crime tipline.
Criminals might proceed anonymously to conceal their participation in a
crime. Anonymity may also be created unintentionally, through the loss
of identifying information due to the passage of time or a destructive
In certain situations, it is illegal to remain anonymous. In the United States, 24 states have “Stop and identify” statutes that requires persons detained to self identify when requested by a law enforcement officer. via wikipedia