Origins
The word 'glossary' preserves within itself a remarkable story about the margins of manuscripts and the history of reading. It comes from Latin 'glossarium' (a collection of glosses), from 'glossa' (an unusual or foreign word requiring explanation), borrowed directly from Greek 'glossa' (γλῶσσα). And it is the Greek word that holds the key, because 'glossa' originally meant something entirely physical: the tongue — the muscular organ in the mouth.
From 'tongue' the word extended to 'language' (your tongue is the language you speak, just as in the English expression 'mother tongue'). And from 'language' it narrowed to something more specific: a word from a foreign or archaic language that an ordinary reader would not understand. Such words needed explanation, and that explanation was called a 'gloss.' The journey from organ of speech to obscure vocabulary item is one of the more charming semantic pilgrimages in etymological history.
In the medieval manuscript tradition, glosses were brief explanatory notes written between the lines or in the margins of texts. When a monk copying a Latin text encountered a word he suspected his readers might not know, he would write a simpler synonym or a brief explanation above or beside it. These interlinear and marginal glosses were the medieval equivalent of footnotes — little acts of scholarly generosity scattered across the pages of hand-copied books. Over centuries, monasteries accumulated vast collections of glosses, and when these were gathered into organized lists, the result was a 'glossarium' — a glossary.
Old English Period
The earliest surviving glossaries date from the 7th and 8th centuries. The Leiden Glossary, compiled around 800 CE probably at the Abbey of Saint Gall, contains Latin words explained with simpler Latin or Old English equivalents. The Epinal-Erfurt Glossary, from around the same period, is one of the oldest surviving examples of written Old English, preserved precisely because monks needed to explain difficult Latin words to English-speaking students. Glossaries were thus not merely reference tools but bridges between languages and between centuries — living records of what readers found difficult and how teachers helped them.
The related word 'gloss' developed an entirely separate and somewhat contradictory life. While a 'gloss' in the scholarly sense meant an honest explanation, the word also came to mean a deceptive interpretation — to 'gloss over' something is to provide a misleadingly smooth surface, covering up difficulty rather than explaining it. This negative sense may have arisen from the suspicion that translators and commentators sometimes twisted texts to serve their own purposes, or it may represent a convergence with a different 'gloss' — the one meaning 'shine, luster,' from a Scandinavian source, which carries the idea of a deceptive surface brilliance.
The Greek root 'glossa' has also given English several other words. 'Polyglot' (speaking many tongues), 'epiglottis' (the flap above the tongue-opening in the throat), and 'glossolalia' (speaking in tongues, from 'glossa' plus 'lalein,' to talk) all carry the original Greek 'tongue' meaning. In biology, 'glossa' names the tongue-like structures in insect mouthparts. The word has been flexible enough to serve anatomists, entomologists, theologians, and lexicographers — all because the Greeks used the same word for a body part and the language that body part produced.
Modern Usage
Today, glossaries remain essential companions to specialized texts. Every textbook, technical manual, and academic treatise benefits from one. The glossary acknowledges what the medieval monks knew: that no reader comes to a text knowing every word, and that the generous act of explaining difficult terms is as old as writing itself. The word 'glossary' carries this tradition in its etymology — a collection of tongues made plain, a gathering of strangeness made familiar.