Psionic energy is a type of energy associated with mental processes, especially telepathy.
Among the unexplained phenomena associated with the galactic barrier is that it can generate an extreme accelleration in psionic ability in some humans. Gary Mitchell is the most notorious example. (TOS: "Where No Man Has Gone Before")
The non-corporeal lifeform known as Onaya stimulated the creativity of other beings using psionic energy. She also left behind traces of psionic energy in her environment that persisted for a few minutes, which could be detected by tricorders. (DS9: "The Muse")