In William Sprague’s book, Lectures on Revivals of Religion, he begins the subject’s treatment by giving a general definition of “revival”–

special occasions on which the agency of the Spirit would be signally manifest (p. 2)

With respect to revivals of religion, Sprague then gives an excellent paragraph on the nature of religion:

Religion consists in a conformity of heart and life to the will of God. It consists in a principle of obedience implanted in the soul, and in the operation of that principle in the conduct…Wherever there exists a cordial belief of God’s truth, and submission of the will to his authority, and the graces of the heart shine forth in the virtues of the life, there is true religion; whether it be in the palace or the cottage; whether it appear in a single individual, or be diffused over a whole community (pp. 6-7).

If this constitutes true religion, a revival of it involves

a revival of scriptural knowledge; of vital piety; of practical obedience…it is moreover applicable, in a strict sense, to the condition of Christians, who, at such a season, are in a greater or less degree revived; and whose increased zeal is usually rendered instrumental in the conversion of sinners. Wherever then you see religion rising up from a state of comparative depression to a tone of increased vigor and strength; wherever you see professing Christians becoming more faithful to their obligations, and behold the strength of the church increased by fresh accessions of piety from the world; there is a state of things which you need not hesitate to denominate a revival of religion (pp. 7-8).

Something we must definitely pray for and work toward!