/kəmˈpɛndiəm/
[noun] A collection of detailed information about a subject, gathered together in one book or document. It can also refer to a set of different games or activities packaged together in one box.
Compendium literally means 'what is weighed together' — the Romans imagined knowledge as something you could balance on a scale.
The word compendium arrives in English from classical Latin, where it carried the vivid meaning of 'a saving' or 'a shortcut' — essentially, the amount saved by cutting something short. The Latin root is the verb compendere, built from the prefix com- (meaning 'together') and pendere (meaning 'to weigh' or 'to hang'). The original Roman mind pictured gathering and weighing things together on a set of scales, which gave the word its earliest sense of balancing or summing up.
In ancient Rome, a compendium was not quite what we imagine today. It referred to a profit, a saving of time or effort, or an abridgement — the idea being that instead of reading everything, you could find what had already been weighed and measured for you. Latin compendium → Late Latin compendium (retaining its sense of abridgement and summary) → English compendium, entering the language in the late 16th century, where it initially meant a concise summary or an abridged account of a larger work.
The root pendere is one of the most productive roots in the entire Latin language, and it hides inside dozens of everyday English words. Latin pendere → Old French pendre → English pendant (something hanging). Latin pendere → Old French peser → English poise (to balance). Latin appendere → Old French apendre → English append and appendix (something hung on). Latin expendere → Old French espendre → English spend and expense (to weigh out money). Latin dispensare → Old French despenser → English dispense and dispensary (to weigh out portions). Even the word pension comes from Latin pensio, the act of weighing out a payment.
When compendium entered English in the 1590s, educated writers used it to describe a summary of a scholarly subject — a book that saved the reader effort by gathering the essential knowledge in one place. It appeared in titles of scientific and theological works, promising the reader a convenient shortcut through a vast body of learning. Over the following two centuries, the word gradually expanded its meaning from an abridgement to a comprehensive collection — almost the opposite direction, shifting from 'the shortest version' to 'the fullest gathering.'
This shift is wonderfully counterintuitive. A word that began its life meaning a time-saving shortcut became associated with thorough, even exhaustive, collections. By the 18th and 19th centuries, a compendium was more likely to suggest a rich, authoritative reference work than a quick summary. The word also found a charming second life in the world of games and leisure, where a compendium describes a box containing multiple board games or puzzles — a collection of entertainments weighed together in one convenient package, preserving at least a ghost of that original Roman sense of gathering many things into one useful whole.