Pneuma (πνεύμα) is an ancient Greek word for "breath," and in a religious context for "spirit" or "soul." It has various technical meanings for medical writers and philosophers of classical antiquity, particularly in regard to physiology, and is also used in Greek translations of the Hebrew Bible and in the Greek New Testament. In classical philosophy, it is distinguishable from psyche (ψυχή), which originally meant "breath of life", but is regularly translated as "spirit" or most often "soul".

Pneuma, "air in motion, breath, wind," is equivalent in the material monism of Anaximenes to aer (ἀήρ, "air") as the element from which all else originated. This usage is the earliest extant occurrence of the term in philosophy. A quotation from Anaximenes observes that "just as our soul (psyche), being air (aer), holds us together, so do breath (pneuma) and air (aer) encompass the whole world." In this early usage, aer and pneuma are synonymous.

In ancient Greek medicine, pneuma is the form of circulating air necessary for the systemic functioning of vital organs. It is the material that sustains consciousness in a body. According to Diocles and Praxagoras, the psychic pneuma mediates between the heart, regarded as the seat of Mind in some physiological theories of ancient medicine, and the brain.