Telepathy, mental combat and psychic powers -- psionics is a catchall word that describes special mental abilities possessed by various creatures. These are spell-like abilities that a creature generates from the power of its mind alone -- no other outside magical force or ritual is needed. The most well known of the psionic creatures is the dreaded mind flayer, which blasts its prey's mind and then devours the brain of the prey while it lies stunned. Each psionic creature's description in the Monster Manual contains details on its psionic abilities.
Psionic attacks almost always allow Will saving throws to resist them. The saving throw (if any) against a psionic ability is 10 + the level of the spell the ability resembles or duplicates + the creature's Cha modifier. However, not all psionic attacks are mental attacks. Some psionic abilities allow the psionic creature to reshape its own body, heal its wounds, or teleport great distances. Some psionic creatures can see into the future, the past, and the present (in far-off locales) as well as read the minds of others. Psionic abilities are usually usable at will.