PSI are psychic powers that are the mainstay of the Mother series. They are wide ranged, and include anything from a raw beam of psychic energy to leaving your opponent more open to attack. They can be used by Ninten, Ana, Ness, Paula, Lucas, and Kumatora, as well as the Maggypsies.
In Mother 2/Earhbound and Mother 3, the main character has a specific PSI attack that only they can use, the moves have default names (though the name for the move can be chosen in the start of the game.)
In Mother 2/Earthbound, Ness's is PK Rockin', a vibrant array of colored psychic beams attack the opponent.
In Mother 3, Lucas's is PK LOVE, an array of ice blue rays siege the opponent.