What is "illithid"

In the Dungeons & Dragons fantasy role-playing game, illithids (commonly known as mind flayers) are monstrous humanoid aberrations with psionic powers. In a typical Dungeons & Dragons campaign setting, they live in the moist caverns and cities of the enormous Underdark. Illithids believe themselves to be the dominant species of the multiverse and use other intelligent creatures as thralls, slaves, and chattels.