百科全书是概要介绍人类的全部知识或某一类知识的工具书。按照辞典形式编排，以条目为基本单元，收集各知识领域的名词、熟语、地名、事件、人物、著作等等，但不收录词语，故不同于语文辞典。

百科全书可以是综合性的，包含所有领域的相关内容（例如《大不列颠百科全书》就是一部著名的综合性百科全书）。也可以是专业性的百科全书（例如医学或哲学百科全书）。也有一些从某种特定文化或国家观点来讨论广泛议题的百科全书，例如《苏联大百科全书》。有时，百科全书也被视为是一个国家和一个时代科学文化发展的标志。

An encyclopedia or encyclopaedia (also spelled encyclopædia, see spelling differences) is a type of reference work or compendium holding a comprehensive summary of information from either all branches of knowledge or a particular branch of knowledge. Encyclopedias are divided into articles or entries, which are usually accessed alphabetically by article name. Encyclopedia entries are longer and more detailed than those in most dictionaries. Generally speaking, unlike dictionary entries, which focus on linguistic information about words, encyclopedia articles focus on factual information concerning the subject for which the article is named.