Anonymity describes situations in which the acting person's identity is unknown. Anonymity may be created unintentionally through the loss of identifying information due to the passage of time or a destructive event, or intentionally if a person chooses to withhold their identity.
There are various situations in which a person might choose to remain anonymous. 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. In many other situations (like conversation between strangers, or buying some product or service in a shop), anonymity is traditionally accepted as natural.