The jump factor per the late great J Gordon Holt:
"Realism," "presence," and "aliveness" are three overworked terms which baffle many readers, and for good reason — they have no definitions, except in terms of "the real thing." And to someone who doesn't remember the sound of live, unamplified instruments or voices, [How can one forget? Aren't these sounds around all the time?] "the real thing" is an equally meaningless term. To one (like myself) whose reference is live sound, realism, presence, and aliveness (interchangeable terms) describe the feeling that I am listening to actual, in-the-flesh instruments rather than their reproduction. This feeling — and it is only that — is brought on by a certain combination of aural cues which my perceptions identify as characteristic of live music. But aliveness does not pertain exclusively to music reproduction; it includes the reproduction of such incidental sounds as page turnings, chair squeakings, and, during a quiet passage, the muted clearing of a performer's throat. The most apt description of aliveness that I have heard — attributed to Sheffield Records' Doug Sax — is "jump factor": Listening to music with only half your attention, you hear a noise from the speakers that sounds so much like it's in the room with you that you jump with surprise.