Mastering the language of medicine is hard enough today, but consider what a medical student in medieval Europe had to contend with. Each Latin technical term in his manuscript textbook might have a host of variant spellings, pronunciations and mistranscriptions. It might be accompanied by words in Latin, Arabic, Greek and vernaculars that claimed to have the same meaning, but might signify something quite different in another treatise or country. Illustrations, if present, rarely clarified matters. Indexing was haphazard, and alphabetization erratic.

In the late thirteenth century, Simon of Genoa (Simon Januensis) undertook the task of putting some order into the chaos of medical terminology. For more than thirty years the papal physician worked on his medical dictionary known as Clavis sanationis (Key of Healing) or, more simply, Synonyma (Synonyms). By the time he finished, sometime before 1296, Simon had written thousands of brief alphabetical entries – almost 1,800 just for the letters A to C.

Simon's primary goal was to explicate the words for diseases; medical instruments; therapies; anatomical parts; and above all, for the plants, animals and minerals used as simple drugs. He generally provided Greek and Arabic equivalents and etymologies for Latin terms. For medicinal substances, he often gave indications of pharmacological properties. He cited almost ninety authorities, including old books in unintelligible scripts that he had discovered in monastery libraries. Even more remarkable was his zeal in checking those written sources with his contemporaries. He wrote to Roger Bacon in England for information, and he travelled widely to interview both men and women for their first-hand linguistic and botanical observations.

Simon's Clavis was among the earliest medical books to make it into print, but it has never had a scholarly edition. To remedy that lack, Barbara Zipser had the brilliant idea of crowd-sourcing the task. With the help of a Wellcome Trust grant, she launched Simon Online as an open-access website in 2011 (www.simonofgenoa.org). Simon Online offers digital images of the Venice 1510 edition (Wellcome copy), with an invaluable transcription of the text by Thomas Smith, and indexes to, among other materials, Simon's first-person remarks and the names of medicinal plants. Even though only a small proportion of the Clavis entries have been edited, translated or commented upon so far, it is clear that twenty-first-century readers are finding their way to Simon's medical lexicon in far greater numbers than he could possibly have imagined.

The first fruits of Simon Online are a 2012 conference, also organized by Zipser, and now the nine papers in this volume. All the papers are notable for their deep knowledge of early medicine and philology, and they amply demonstrate how effective collaborative Internet-based scholarship can be.

Vivian Nutton and Peregrine Horden describe Simon's work in the context of medieval medicine generally and the papal court in particular. Petros Bouras-Vallianatos analyses the pharmacological content of Simon's Clavis and its utility as a reference work to medieval readers. Siam Bhayro tackles the difficult linguistic questions: how much Arabic did Simon know? Where had he learnt it? What help did he have? The contributions by Marie Cronier, Caroline Petit, Valerie Knight and Charles Burnett assess Simon's use of four key medical authors: Dioscorides and Galen from the first century; Alexander of Tralles from sixth-century Byzantium; and a shadowy twelfth-century figure, Stephen, ‘the Disciple of Philosophy’, who translated Haly Abbas's Arabic treatise on medical theory and practice into Latin. Simon criticized Stephen's trilingual glossary (Greek, Arabic, Latin), but nonetheless relied heavily on it in the Clavis. Concluding the volume, Barbara Zipser comments on the ways visitors – from classical scholars to practising herbalists – are tapping the resources offered by Simon Online.

Simon of Genoa's Medical Lexicon does not pretend to be the definitive book on Clavis sanationis, but I fervently hope that this is just the first of many conferences and books on the medical–linguistic worlds that Simon represents. Meanwhile, the door is open at Simon Online for anyone who wants to join in the fun.