The Latin words machina and organum can both be translated by "machine", but in the beginning of book X, Vitruvius makes a distinction between the two words according to two criteria: the number of operators (the machina is worked by several operators, the organum requires skill). In any case, the distinction does not refer to a difference between a compound machine and a simple machine, as it has sometimes been said. The object of this article is to examine if the distinction made by Vitruvius corresponds to the state of the language in the first century B.C., or if it is due to an impression proper to the author of De architectura ; and whether this distinction can explain the big divergence in the history of the two words, from Latin to modern languages : "machine" has kept a general meaning, "organ" refers to a specific musical instrument. We will examine in detail the examples given by Vitruvius to make a light on the distinction he made. They will be compared to other uses of these words by Vitruvius himself and other writers. Some contradictions appear in Vitruvius’ own texts, but a repartition of uses that are in some aspects consistent with Vitruvius’ theories, can be observed in the language. That repartition explains the divergence in the semantic evolution of the two words.