According to Darth Plagueis' writings, pneuma was the part of the Force that was the expression of conscious thought. It was one of the three aspects of the Force, along with the aperion and the anima.
Thinking, self-aware minds contributed to the collective pneuma, and it could be accessed by naturally telepathic species, as well as the various mind tricks by both the Sith and the Jedi.
This aspect (or category) of existence that draws on the Force was first mentioned in the Book of Sith: Secrets from the Dark Side, on page 130.
This aspect (or category) of existence that draws on the Force was first mentioned in the Book of Sith: Secrets from the Dark Side, on page 130.