Anonymous

adj. nameless, of an unknown person

Anonymity is derived from the Greek word ανωνυμία, 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".
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Adjective
1. having no known name or identity or known source; "anonymous authors"; "anonymous donors"; "an anonymous gift"
(synonym) anon.
(antonym) onymous
(similar) nameless, unidentified, unknown, unnamed
2. not known or lacking marked individuality; "brown anonymous houses"; "anonymous bureaucrats in the Civil Service"
(similar) faceless

(a.)
Nameless; of unknown name; also, of unknown or unavowed authorship; as, an anonymous benefactor; an anonymous pamphlet or letter.
  

noun
organization of former alcoholics which helps sufferers from alcoholism to overcome their dependence on alcohol by encouraging them to talk about their problems in group therapy