C.S. Lewis usually used the name Jack among friends.
His full name was Clive Staples Lewis, but “Jack” became his familiar personal name. The nickname originated after the death of his childhood dog, Jacksie, when Lewis began calling himself “Jacksie” and later shortened it to Jack.
The name was used by his family, school friends, fellow academics, and literary associates. It is especially common in biographies and memoirs about Lewis, where “Jack” distinguishes the private man from the public author known as C.S. Lewis.
Lewis’s initials came from his formal name: Clive Staples Lewis. “C.S. Lewis” became the name under which he published fiction, apologetics, literary criticism, and devotional works. Readers sometimes assume that Jack was a pen name, but it was instead the personal name he adopted in childhood and retained throughout his life.