a reference work (often in several volumes) containing articles on various topics (often arranged in alphabetical order) dealing with the entire range of human knowledge or with some particular specialty
"My grandfather spent his entire retirement compiling notes for our family encyclopedia, which eventually covered everything from local genealogy to the history of every farm in our county."
A comprehensive reference work (often spanning several printed volumes) with articles (usually arranged in alphabetical order, or sometimes arranged by category) on a range of subjects, sometimes general, sometimes limited to a particular field.
"After pouring through the entire encyclopedia from cover to cover, I finally found the detailed entry on quantum physics that my professor had recommended."
In plain English: An encyclopedia is a large collection of books or an online website that contains facts and information about many different topics.
"We spent hours looking through the encyclopedia to find information about dinosaurs."
Usage: An encyclopedia is a large collection of detailed information on many subjects, traditionally found as a multi-volume set but now also available digitally. Use this noun when referring to a comprehensive reference work that provides articles arranged alphabetically or by category to cover a wide range of topics.
The word encyclopedia comes from the Greek phrase for "general education," which literally meant learning within a circle of arts and sciences. While its roots are ancient, the modern English usage was largely shaped by the influential French reference work published by Diderot and his team.