One of the most widely-used reference tools for students and others is the encyclopedia. By definition, an encyclopedia is a collection of knowledge, either on all topics or specific subjects. In past years encyclopedias were printed volumes, with many volumes sometimes needed in order to make up a set. These are still available on a limited basis, but most encyclopedias are found on CD-ROM or online today.
Encyclopedias have a long history. The first recorded version of a compendium of knowledge was the "Naturalis Historia" of Pliny the Elder, which dates from around 77 AD. In the Middle Ages, encyclopedias focused on religious issues and the seven liberal arts - grammar, rhetoric, logic, mathematics, geometry, music and astronomy. Muslim scholars of the era also wrote encyclopedias on science, medicine and sociology.
All of these early encyclopedias shared the same difficulty of distribution, since they were created before the invention of movable type printing presses. Copies were handwritten and of very limited distribution. In the 18th Century and later times, the wide availability of cheap printing and paper combined with explosive growth in discovery and learning led to a huge number of encyclopedias, culminating in the Encyclopedia Brittanica, which was published in updated editions throughout the 19th Century. The encyclopedia reached a peak of popularity in the 20th Century, with the World Book and Funk and Wagnall's Encyclopedia appearing in millions of homes.
In the modern era, encyclopedias are available in a number of digital formats. This process began with the "Encarta" series produced on CD-ROM by Microsoft, and continued to grow in online formats. Today countless users access Wikipedia and the digital versions of Encyclopedia Brittanica on a daily basis. The major difference is that users have the ability to change Wikipedia and update it, therefore becoming encyclopedia writers themselves.