Definition of "esper"

I don't know why anybody would be spying on me, either, but—I'm neither a Lensman nor an esper, but I'd swear that somebody's peeking over my shoulder half the time.

1942 June, Edward Elmer Smith, “Storm Cloud on Deka”, in Astonishing Stories‎, volume 3, number 4

But Espers can’t pick up any EK brain pattern farther than the horizon. It’s direct wave transmission. Even large objects cannot be penetrated. A building, say, or a—

1950 August, Alfred Bester, “The Devil's Invention”, in Astounding Science Fiction‎, volume 45, number 6, page 139