Charles Buck Theological Dictionary 
The chief ruler of a monastery or abbey. At first they were lay-men, and subject to the bishop and ordinary pastors. Their monasteries being remote from cities, and built in the farthest solitudes, they had no share in ecclesiastical affairs; but, there being among them several persons of learning, they were called out of their deserts by the bishops, and fixed in the suburbs of the cities; and at length in the cities themselves. From that time they degenerated, and, learning to be ambitious, aspired to be independent of the bishops, which occasioned some severe laws to be made against them. At length whoever, the abbots carried their point, and obtained the title of lord, with other badges of the episcopate, particularly the mitre. Hence arose new distinctions among them. Those were termed mitred abbots who were privileged to wear the mitre, and exercise episcopal authority within their respective precincts, being exempted from the jurisdiction of the bishop. Others were called crosiered abbots, from their bearing the crosier, or pastoral staff. Others were styled aecumenical or universal abbots, in imitation of the patriarch of Constantinople, while others were termed cardinal abbots, from their superiority over all other abbots. At present, in the Roman Catholic countries, the chief distinctions are those of regular and commendatory. The former take the vow and wear the habit of their order; whereas the latter are seculars, though they are obliged by their bulls to take orders when of proper age.
Webster's Dictionary 
(1): (n.) One of a class of bishops whose sees were formerly abbeys.
(2): (n.) The superior or head of an abbey.
Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
The Nuttall Encyclopedia 
Head of an abbey. There were two classes of abbots: Abbots Regular, as being such in fact, and Abbots Commendatory, as guardians and drawing the revenues.