An encyclopedia is a type of reference work that provides summaries of information on various topics. It is divided into articles or entries that are usually accessed alphabetically. Some key characteristics of encyclopedias include their subject matter, scope, hierarchical organization, and production by subject experts. The first major encyclopedias date back to ancient times, but the modern printed encyclopedia emerged in the 18th century. In the digital age, encyclopedias have been published online and as CD-ROMs, allowing for new methods of organization and updating content.