Dhikr is remembrance: the practice of calling God to mind, usually by repeating His name or a short formula, until the repeating takes up the whole attention. Nicholson calls it the keystone of practical religion for the mystics, and explains the word and its history in a few lines:
The term dhikr--‘recollection’ seems to me the most appropriate equivalent in English--signifies ‘mentioning,’ ‘remembering,’ or simply ‘thinking of’; in the Koran the Faithful are commanded to “remember God often,” a plain act of worship without any mystical savour.
The Sufis made something more of it: “a practice of repeating the name of God or some religious formula,” he says, “accompanying the mechanical intonation with an intense concentration of every faculty upon the single word or phrase.” They valued this “irregular litany” above the five daily prayers, because it allowed them uninterrupted communion. (He also notes, in his introduction, that litanies and vows of silence may have Christian precedents; Introduction.)
The fullest instruction in the sources here is al-Ghazālī’s, in a passage Nicholson quotes as summarised by D. B. Macdonald. The seeker sits alone, lightens his other duties, and keeps his mind on one thing:
“Then, as he sits in solitude, let him not cease saying continuously with his tongue, ‘Allah, Allah,’ keeping his thought on it. At last he will reach a state when the motion of his tongue will cease, and it will seem as though the word flowed from it.”
The practice then moves inward. The tongue falls silent, and then even the shape of the word goes: “until the form of the word, its letters and shape, is removed from his heart, and there remains the idea alone, as though clinging to his heart, inseparable from it.” The text is exact about where effort stops. “So far, all is dependent on his will and choice; but to bring the mercy of God does not stand in his will or choice.” The seeker’s part is to lay himself bare and wait.
Nicholson adds a story of a master who set a disciple to say “Allah! Allah!” all day, then all night, “until they came forth from his lips even while he was asleep,” and then to be silent and recollect them, until “the disciple’s whole being was absorbed by the thought of Allah.” Aids were various; he mentions the breathing exercises known from India and the use of music and movement to induce the state of passing-away.
One Sufi, quoted in the same chapter, states the goal of the practice in a single sentence:
“The first stage of dhikr is to forget self, and the last stage is the effacement of the worshipper in the act of worship, without consciousness of worship, and such absorption in the object of worship as precludes return to the subject thereof.”
Al-Hujwīrī builds the same idea into his definition of the two great terms. In his account, annihilation “means ‘annihilation of all remembrance of other’, and subsistence means ‘subsistence of the remembrance of God’ (baqá-yi dhikr-i ḥaqq).” (Chapter XIV; see Fanā and Baqā.) In the Awārif, Suhrawardī’s disciple keeps “the tongue in gikr (remembrance of God), the heart in fikr (thought of God), and the soul in mushahida (viewing God’s glory)” until he “considereth himself non-existent.” (Introduction)
The tradition does not confine dhikr to the cell. Al-Hujwīrī records a master at Mecca, asked for “a lawful thing in which there is nothing unlawful.” His answer: “Praise (dhikr) of God is a lawful thing in which there is nothing unlawful, and praise of aught else is an unlawful thing in which there is nothing lawful.” (Chapter X) Al-Ghazālī, in the Alchemy of Happiness, treats constant remembrance as the sign of love: “the remembrance of God should always remain fresh in a man’s heart without effort, for what a man loves he constantly remembers.” (Chapter VIII) In an earlier chapter he gives the prophetic saying that frames the whole subject: “The world is a curse, and all which is in it is a curse, except the remembrance of God, and that which aids it.” (Chapter III)
Related entries: Samāʿ (the audition that often accompanies dhikr), Nafs and Rūḥ (what the remembering is meant to quiet), and Qabḍ and Basṭ (the states the practice can neither summon nor prevent).
The passages above are quoted from complete public-domain translations, all of which are free to read here in full.
Read The Mystics of IslamRead the Awārif al-Maʿārif