The wiccan rede (pronounced reed) is the central moral principle of most wiccans.
The wiccan rede is: "if it harms none, do what you will".
The rede is most often written with archaic english words, as "an it harm none, do what ye will", which is the form in which the rede was first publicized.
The rede was created by Gerald Gardner, shortly before his death. Earlier, Gardner had created a precursor to the wiccan rede in his wiccan laws of 1957; that precursor was the simple phrase "harm none". Gardner created a second precursor to the wiccan rede in his 1959 book The Meaning of Witchcraft, in chapter 8, in which he stated that witches follow the moral principle of "do what you like so long as you harm no one". Gardner told the rede to Doreen Valiente, who later publicized the rede in a speech that she gave on october 3rd, 1964, after Gardner had died earlier that year. In the speech, Valiente said: "Eight words the wiccan rede fulfill: An it harm none, do what ye will.". Shortly after that speech, the wiccan rede was publicized in multiple neo-pagan periodicals.
The word "rede" derives from middle english, and means "advice" or "counsel". It is closely related to the german word rat or råd.
"An" is an archaic middle english conjunction, meaning "if". "Ye" is an archaic or dialect form of "you" (plural).
The wiccan rede is sometimes altered by replacing the phrase "ye will" with "thou wilt", in imitation of Aleister Crowley's saying "do what thou wilt".
The Crowleyanized wiccan rede thus reads: "An it harm none, do what thou wilt.".
That Crowleyanized version of the wiccan rede was invented and publicized by Raymond Buckland,
via his 1986 book Buckland's Complete Book of Witchcraft.
Crowley in turn got the phrase 'thou wilt' from the Hermetic Order of the Golden Dawn, to which he belonged.
There is also a "long wiccan rede" or "long rede", which is based upon the original wiccan rede, but is far longer.
For information about the long wiccan rede, see the article long wiccan rede.