An encyclopaedia is a systematic summary of the knowledge that is most important to man-kind. It is a work having information on all subjects or limited to a special field or subject, arranged in systematic (generally alphabetical) order. Encyclopaedias might be in one volume, in which case very small information will be given, or they any be in many volumes in which the various kinds of matter will be comprehensive. Encyclopaedias are usually written by experts, and sometimes contain bibliographies and illustrations. It is different from dictionaries in the sense that dictionary, tells "what", about a word whereas an encyclopeadia tells "what","how", "when", "where'', and "why" of an idea, a place, a person, air event or things.
Encyclopaedias are major reference sources, containing so much information that at one time, good encyclopaedias were referred to as the backbone of the reference service in the libraries. The etymology of the word encyclopaedia is Greek and means a cycle of instruction, which otherwise means good education. The term was first used in the book Johann Henrich Alsted's Encyclopaedia Cursus Philosophici, Herbom, 1608.
The first known encyclopaedia was written by Aristotle, the ancient Greek philosopher, which resulted from his assembled treatises. That is why Aristotle is referred to as the father of encyclopaedias although he never intended to write one. The first encyclopaedia to be published in English was John Harris's Laxion Technicum, or, An Universal English Dictionary of the Arts and Sciences, London, 1704. One of the' initial encyclopaedias was the Spanish Archbishop Isidore of Seville's Etymologiarum sine originum libriXX which was completed in 623 A.D. More than a thousand manuscripts of this survived, and in printed form it had an undiminished appeal as late as the 17th century.
Encyclopaedias are of various types. Two main types are the General encyclopaedia such as the Encyclopaedia Britanica and the subject encyclopaedias like the Encyclopaedia of Religion.