Psi (Psionic Energy) is an energy field linking together all of the minds within The Universe into a semi-cohesive system that only a rare few people can perceive and utilize. It has existed in some variation before the current Universe but Psi as it is currently understood was created at the birth of The Universe alongside Mana.
Psi is present in all minds from sentient to non-sentient creatures, but it requires a biological element in order to manifest meaning that mechanical entities cannot utilize it, nor can anyone using Psi attempt to access their artificial mind. Biomechanical Androids however can manifest Psi as they possess biological brains.
Rarely creatures or species can exist that are almost entirely composed of Psi, in a similar manner to how Majin are created from Mana. Being composed of psionic energy grants them a variety of abilities and strengths above ordinary individuals composed of matter.