Zikr is an Arabic word; it means remembrance. Zikrullah, then, means simply to remember God. It is not a formal act, but rather a spontaneous one, which comes as naturally to one who has come to know God as singing does to a bird.
A spiritual upheaval of the utmost intensity occurs in one who discovers God in all His power and glory. Suddenly, God is forever in one's heart and thoughts. One's constant remembrance of God expresses itself in multifarious forms.
Sometimes it is an inward experience — a tingle of joy, a shiver of fear that creeps down one's body as thoughts of God fill one's mind. Sometimes one enters into a spontaneous outpouring of thanksgiving and adoration. It is this state of mind that constitutes remembrance of God, whether it is expressed in the form of words of praise or silent thoughts.
Sometimes one looks at outer space in all its infinite vastness and ponders on the stars and constellations spread out there. "How great must be the Lord who has arranged this marvellous display and runs it with such superb finesse," such is one's reaction to the sight spread out before one.
Sometimes one gazes at rivers, trees and mountains, and one's heart is touched by their beauty, by the very meaningfulness of their existence. If one has discovered God, everything around one reminds one of Him, sparking off a never-ending litany of remembrance in one's mind and heart. Then one will look critically at oneself, and realise one's own errors and shortcomings.
Moved to seek the Lord's forgiveness, one will pray to Him for salvation from eternal punishment: "Lord, admit me into the shade of Your mercy on that day when there will be no other place to take refuge."