ISP or I.S.P. is the abbreviation of Inner Strength Points.
Psychic/Psionic characters can tap into their own Inner Strength of their mind to move objects (Physical), read minds (Sensitive), heal with a touch (Healing) or even possess another person (Super)
Anybody can have Psionic abilities.
Cybernetics & Bionics:
Cybernetics do not invade the braincase, they rely on picking up nerve impulses at the point of prosthesis connection to operate the cybernetic implant.
Bionics include brain implants to operate the prosthetics, and these weaken the homeostasis of the brainby introducing foreign objects, and thus reduce the person's ISP pool.
A Partial Reconstruction borg will lose half of his ISP, whereas a Full Reconstruction borg has his ISP completely obliterated. (Rifts, p.48)