An encyclopedia (also known in English as an encyclopædia) is a collection (usually a book or website) of information. Some are called "encyclopedic dictionaries".[1][2][3][4]
All encyclopedias were printed, until the late 20th century, when some were on CDs and the Internet. 21st century encyclopedias are mostly online by Internet. The largest encyclopedia in the English language is English Wikipedia, which has more than 6 million articles. The second largest is the Encyclopædia Britannica, which is the largest one that is printed. Either kind of encyclopedia can inform us on many different topics.
Book series were used to summarize all knowledge have been published for thousands of years. A famous early one was the Natural History by Pliny the Elder. The name "encyclopedia" is from the 16th century and meant "complete knowledge". The French Encyclopédie of Denis Diderot was the first that had major parts written by many people from all around the world.
After the printing press was invented, dictionaries with long definitions began to be called encyclopedias that were books that has articles or subjects For example, a dictionary of science, if it included essays or paragraphs, it 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 such as Britannica were started for the purpose of publishing encyclopedias for sale to individuals, and for public use in libraries. Like dictionaries (which had definitions), these publishers hired hundreds of experts to write articles and read and choose articles. Some internet encyclopedias allowed their paying customers to submit articles from other encyclopedias. Other internet encyclopedias accepted writing from non-paying users (users who did not sign in) of the encyclopedia.