Hermeticism or Hermetism is a philosophical and religious system based on the purported teachings of Hermes Trismegistus (a Hellenistic conflation of the Greek god Hermes and the Egyptian god Thoth). These teachings are contained in the various writings attributed to Hermes (the Hermetica), which were produced over a period spanning many centuries (300 BCE – 1200 CE) and may be very different in content and scope.
One particular form of Hermetic teaching is the religio-philosophical system propounded by a specific subgroup of Hermetic writings known as the 'religio-philosophical' Hermetica, the most famous of which are the Corpus Hermeticum (a collection of seventeen Greek Hermetic treatises written between c. 100 and c. 300 CE) and the Asclepius (a treatise from the same period mainly surviving in a Latin translation). This specific, historical form of Hermetic philosophy is sometimes more restrictively called Hermetism, to distinguish it from the philosophies inspired by the many Hermetic writings of a completely different period and nature.
A more open-ended term is Hermeticism, which may refer to a wide variety of philosophical systems drawing on Hermetic writings, or even merely on subject matter generally associated with Hermes (most notably, alchemy often went by the name of "the Hermetic art" or "the Hermetic philosophy"). The most famous use of the term in this broader sense is in the concept of Renaissance Hermeticism, which refers to the wide array of early modern philosophies inspired by, on the one hand, Marsilio Ficino's (1433–1499) and Lodovico Lazzarelli's (1447–1500) translation of the Corpus Hermeticum, and on the other, by Paracelsus' (1494–1541) introduction of a new medical philosophy drawing upon the 'technical' Hermetica (i.e., astrological, alchemical, and magical Hermetica, such as the Emerald Tablet).
In 1964, Frances A. Yates advanced the thesis that Renaissance Hermeticism, or what she called "the Hermetic tradition", had been a crucial factor in the development of modern science.