An encyclopedia (or encyclopædia, cyclopædia, encyclopaedia) is a collection (usually a book) of information about things humans know. Whether it is published on paper or online, an encyclopedia is a collection of essays, each on a different subject. The first encyclopedias may have begun as lecture notes by and for the teachers of ancient schools, such as Aristotle’s school. Each volume in the encyclopedia was for a different subject or course of study. The word encyclopedia at first meant the teachings of the school. After the printing press was invented, dictionaries with long definitions began to be called encyclopedias. For example, a dictionary of science, if it included essays, was thought of as an encyclopedia or knowledgeable book on the subject of science. Some encyclopedias then put essays on more than one subject in alphabetical order instead of grouping them together by subject. The word, encyclopedia, was put in the title of some encyclopedias. Companies were started for the purpose of publishing encyclopedias for the public use in libraries, which is different from when an encyclopedia was the curriculum of a private school. Like dictionaries, these publishers hired hundreds of experts to write articles and read and choose articles. Some internet encyclopedias allowed their paying customers to submit articles. Other internet encyclopedias accepted writing from non-paying users of the encyclopedia. Encyclopedias are a great source of knowledge. They are included with many different topics. 
People have made encyclopedias for hundreds of years, but the name "encyclopedia" is from the 16th century.
Wikipedia is an Encyclopedia too.
Types of encyclopedias [change]
There are different types of encyclopedias. Some are general and have pages on lots of topics. The English language Encyclopædia Britannica and German Brockhaus are general encyclopedias. Some are about specific topics. For example, there are encyclopedias of medicine or philosophy. There are also some encyclopedias that have lots of topics with one point of view or one cultural bias. The Great Soviet Encyclopedia is one of these.
Many dictionaries have a differents sort of information to encyclopedias. Examples are the Dictionary of National Biography, the Dictionary of American Naval Fighting Ships, and Black's Law Dictionary.
There are two main ways of organizing encyclopedias: from A to Z (the alphabetical way) or by categories. Most encyclopedias go from A to Z.
There are also printed encyclopedias, and encyclopedias in the computer, such as Wikipedia.
The largest online encyclopedia in the English language is Wikipedia in English, which has more than 4 million articles now. The second largest is the Encyclopaedia Britannica, which is the largest one that is printed. Stacy Schiff, a writer, says that Wikipedia is not as good as other encyclopedias because anyone can change it, so some people may write things that are wrong. Also, the way that Wikipedia works means that there is likely to be bias. On the other hand, Tyler Cowen, an Economist, says that other non-fiction writing may also have the same problems.
- The Compact Edition of the Oxford English Dictionary, Volume I A-O. 1971. Encyclopedia. New York: Oxford University Press.
- Webster’s Third New International Dictionary . . . Unabridged . . . Merriam-Webster. 1961. Encyclopedia. Springfield, MA: G & C Merriam Company.
- Funk & Wagnalls New Encyclopedia, Volume 9. MCMLXXXIII. Encyclopedia. USA: Funk & Wagnalls, Inc.
- The Columbia Encyclopedia in one volume. 1940. Encyclopedia. New York, NY: Columbia University Press.
- Schiff, Stacy (31 July 2006). "Know It All". The New Yorker. http://www.newyorker.com/archive/2006/07/31/060731fa_fact. Retrieved 15 August 2008. "open editing invites abuse. Senators and congressmen have been caught tampering with their entries; the entire House of Representatives has been banned from Wikipedia several times."
- Cowen, Tyler (12 March 2008). "Cooked Books". The New Republic. http://www.tnr.com/story.html?id=82eb5d70-13bd-4086-9ec0-cb0e9e8411b3. Retrieved 15 August 2008. "The sad truth is that "non-fiction" has been unreliable from the beginning, no matter how finely grained a section of human knowledge we wish to consider. For instance, in my own field, critics have tried to replicate the findings in academic journal articles by economists using the initial data sets. Usually, it is impossible to replicate the results of the article even half of the time. Note that the journals publishing these articles often use two or three referees--experts in the area--and typically they might accept only 10 percent of submitted papers."