Etymologiae (Latin for 'Etymologies'), also known as the Origines ('Origins'), usually abbreviated Orig., is an etymological encyclopedia compiled by the influential Christian bishop Isidore of Seville (c. 560–636) towards the end of his life. Isidore was encouraged to write the book by his friend Braulio, Bishop of Saragossa. Etymologiae summarized and organized a wealth of knowledge from hundreds of classical sources; three of its books are derived largely from Pliny the Elder's Natural History. Isidore acknowledges Pliny, but not his other principal sources, namely Cassiodorus, Servius, and Gaius Julius Solinus.

Etymologiae covers an encyclopedic range of topics. Etymology, the origins of words, is prominent, but the work also covers, among other things, grammar, rhetoric, mathematics, geometry, music, astronomy, medicine, law, the church and heretical sects, pagan philosophers, languages, cities, humans, animals, the physical world, geography, public buildings, roads, metals, rocks, agriculture, war, ships, clothes, food, and tools.

Etymologiae was a widely used textbook throughout the Middle Ages. It was so popular that it was read in place of many of the original classics that it summarized; as a result, some of these ceased to be copied and were lost. It was cited by Dante Alighieri (who placed Isidore in his Paradiso), quoted by Geoffrey Chaucer, and mentioned by the poets Boccaccio, Petrarch, and John Gower. Among the thousand-odd surviving manuscript copies is the 13th-century Codex Gigas; the earliest surviving manuscript preserves a fragment of book XI from the 7th century. Etymologiae was printed in at least ten editions between 1472 and 1530, after which its importance faded during the Renaissance. The first scholarly edition was printed in Madrid in 1599; the first modern critical edition was edited by Wallace Lindsay in 1911.

While less well known in modern times, modern scholars recognize the work's importance in preserving both classical texts, as well as insight into the medieval mindset.

Isidore of Seville was born around 560 in Cartagena, and became bishop of Seville. He was widely read, mainly in Latin with a little Greek and Hebrew. He was familiar with the works of both the church fathers and pagan writers such as Martial, Cicero and Pliny the Elder, this last the author of the major encyclopaedia then in existence, the Natural History. The classical encyclopedists had already introduced alphabetic ordering of topics, and a literary rather than observational approach to knowledge: Isidore followed those traditions. Isidore became well known in his lifetime as a scholar. He started to put together the Etymologiae, a collection of his knowledge, in about 600, and continued to write until around 625.

The Etymologiae presents an abbreviated form of much of that part of the learning of antiquity that Christians thought worth preserving. Etymologies, often very far-fetched, form the subject of just one of the encyclopedia's twenty books (Book X), but perceived linguistic similarities permeate the work. An idea of the quality of Isidore's etymological knowledge is given by Peter Jones: "Now we know most of his derivations are total nonsense (eg, he derives baculus, 'walking-stick', from Bacchus, god of drink, because you need one to walk straight after sinking a few)".

The work covers many of the subjects of ancient learning, from theology to the construction and provenance of furniture, and provides a rich source of classical lore and learning for medieval writers. Isidore quotes from around 475 works from over 200 authors in his works, including those outside the Etymologiae. Bishop Braulio, to whom Isidore dedicated it and sent it for correction, divided it into its twenty books.

An analysis of Book XII by Jacques André identifies 58 quotations from named authors and 293 borrowed but uncited usages: 79 from Solinus; 61 from Servius; 45 from Pliny the Elder. Isidore takes care to name classical and Christian scholars whose material he uses: in descending order of frequency, Aristotle (15 references), Jerome (10 times), Cato (9 times), Plato (8 times), Pliny, Donatus, Eusebius, Augustine, Suetonius, and Josephus. He mentions as prolific authors the pagan Varro and the Christians Origen and Augustine. But his translator Stephen Barney notes as remarkable that he never actually names the compilers of the encyclopedias that he used "at second or third hand", Aulus Gellius, Nonius Marcellus, Lactantius, Macrobius, and Martianus Capella. Barney further notes as "most striking" that Isidore never mentions three out of his four principal sources (the one he does name being Pliny): Cassiodorus, Servius and Solinus. Conversely, he names Pythagoras eight times, even though Pythagoras wrote no books. The Etymologiae are thus "complacently derivative".