Fermentum, -i (n) – the cause of fermentation, leavening i.e. yeast
[from ferveo/fervo – to boil, foam, enrage, agitate]
This BCS “Defined” entry is brought to us by Pliny the Elder, whose fascinating discussion on leavened bread introduced us to this delightful Latin word.
Although the causes of leavening in bread and alcoholic production in beer are essentially the same, it seems like this was not necessarily known by the Romans. Fermentum is frequently used for bread, but does not directly refer to beer (to my knowledge) until the 4th-5th c. AD in a passage by Paulus Orosius. In the subsequent Late Antique and Medieval beer texts, however, fermentum is used much more frequently.