Psychic energy, also known as mental energy, is the energy of a sentient's brain. All sentients have psychic energy, but those who are trained can use their mental energy to invoke psionic powers. Those that do this are called mentalists or psionicists.
At Nemexus, the primordial Ingu'lumin created the first mental rendering of writing. This is done using psychic energy, creating what became known as psychic imprints. Reading these requires one to either know telepathy or be a mentalist.