From BiblePortal Wikipedia

Charles Buck Theological Dictionary [1]

Venerable; deserving awe and respect. It is a title of respect given to ecclesiastics. The religious abroad are called reverend fathers; and abbesses, prioresses, &c. reverend mothers. In England, bishops are right reverend, and archbishops most reverend; private clergymen, reverend. In France, before the revolution, their bishops, archbishops, and abbots, were all alike, most reverend. In Scotland, the clergy individually are, reverend; a synod is, very reverend; and the general assembly is, venerable. The Dissenters, also, in England have the title of reverend; though some of them suppose the term implies too much to be given to a mere creature, and that of God only it may be said with propriety, "Holy and reverend is his name, "  Psalms 111:4 .

King James Dictionary [2]

REV'EREND, a. L. reverendus.

1. Worthy of reverence entitled to respect mingled with fear and affection as reverend and gracious senators.

A reverend sire among them came.

This epithet is, I believe, never applied to the Supreme Being, or to his laws or institutions. In lieu of it we use venerable.

2. A title of respect given to the clergy or ecclesiastics. We style a clergyman reverend a bishop is styled right reverend an archbishop most reverend. The religious in catholic countries, are styled reverend fathers abbesses, prioresses, &c. reverend mothers. In Scotland, as in the United States, the clergy are individually styled reverend. A synod is styled very reverend, and the general assembly venerable.

Webster's Dictionary [3]

(a.) Worthy of reverence; entitled to respect mingled with fear and affection; venerable.

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [4]

a title prefixed by courtesy to the name of any clergyman, though "clerk "(clericus) is the legal and strictly proper description of clergymen, and is, in official documents, placed after (as "Reverend" is before) their names. In the Church of England and the Protestant Episcopal and Roman Catholic churches the title is given to ecclesiastics of the second and third orders, the bishops being styled "right reverend." In some churches ordained abbesses and prioresses are called "reverend mothers."

The Nuttall Encyclopedia [5]

A title of respect given to the clergy, Very Reverend to deans, Right Reverend to bishops, and Most Reverend to archbishops.