The word zikr (pl. azkar), zikra or tazikra is derived from z-k-r appear in 274 Koranic verses, means remembrance or recollection. The most important significance of the first form of the verb is "thinking about" or "calling to mind" with the remembrance of God being the primary focus. The Koran says, "and remember God often" (33:40) and "the remembrance of God makes the heart calm" (13:28).
Ali bin Muhammad bin al-Walid (d. 612/1215) writes in his Taju'l Aqa'id (tr. W. Ivanow, Bombay, 1936, pp. 56-7) that, "Prayer (salat) is of two kinds. One is the ordinary prescribed prayer, namaz, which consists of the recitation of prescribed formulas, accompanied by special genuflexions, prostrations, etc. The other form of prayer is spiritual, it is based on high training of the soul and consciousness, and belongs to the sphere of the "worship by knowledge" (al-ibadatu'l ilmiyya). It is differently described by terms, such as "attachment" (ittisal) to God, or "approachment" (taqarrub) to Him, or "union" (ittihad) with Him or "connection with the higher world" (al-irtibat bi'l-mala'il-a'la). It has nothing to do with any fixed formula, or prescribed genuflexions or prostrations, or any special movements of the body. It can only be performed in spirit, by the force of continuous meditation, or by the power of concentration (bi-quwwati'l-irtibad wa shiddati'l muhafaza), by persistent effort to preserve spiritual purity, by keeping away from temptations of one's lower self, abandonment of lust, and exercise of self control (sabr) in the most difficult and unpleasant situations of life, or fatiguing forms of worship. When one masters all this, he has really attained the desired attachment to God (haqiqatu'l ittisal)." Zikr is a strong pillar in the path toward God, nay rather the most important pillar, for nobody can reach God without constantly remembering Him.
Yahya Ahmed writes in Veils and Keys (Kuala Lumpur, 1998, pp. 240-241) that, "A person performs the earthy duties (arkan) of the prayer, the fast, the religious zakat tax and makes the pilgrimage to Makkah first and foremost because they are actions that pull the mind away from the mundane affairs of the world and focus the mind and heart on the one thing worth remembering that God exists as the unifying and one Reality. In addition, the remembrance through the earthy duties offers its own compensations and provide the believer with the blessing and the inner peace that lie as spiritual needs at the root of his being.