The Collaborative International Dictionary of English v.0.48:

Phonograph \Pho"no*graph\, n. [Phono- + -graph.]
   1. A character or symbol used to represent a sound, esp. one
      used in phonography. [archaic]
      [1913 Webster]

   2. (Physics) An instrument for the mechanical registration
      and reproduction of audible sounds, as articulate speech,
      etc. An early simple version consisted of a rotating
      cylinder or disk covered with some material easily
      indented, as tinfoil, wax, paraffin, etc., above which is
      a thin plate carrying a stylus. As the plate vibrates
      under the influence of a sound, the stylus makes minute
      indentations or undulations in the soft material, and
      these, when the cylinder or disk is again turned, set the
      plate in vibration, and reproduce the sound. Modern
      versions use electronic circuitry and various more stable
      recording media to record sound more accurately.
      [1913 Webster + PJC]

   3. An instrument for reproducing sounds, especially music,
      previously recorded on a plastic cylinder or disk as a
      pattern of bumps or wiggles in a groove. A needle (stylus)
      held in the groove is made to vibrate by motion (rotation)
      of the recording, and the vibrations caused by the bumps
      and wiggles are transmitted directly to a membrane, or
      first transduced into electrical impulses and sent to an
      electronic amplifier circuit, thereby reproducing with
      greater or less fidelity the original sounds. A phonograph
      which is equipped with electronics enabling the playback
      of sound with high fidelity to the original is often
      called a hi-fi.

   Note: In the 1990's such devices are beginning to be replaced
         in many homes by compact disk players; the production
         of plastic recordings of music for playback on a
         phonograph has almost ceased for entertainment
         purposes.
         [PJC]

The Collaborative International Dictionary of English v.0.48:

hifi \hifi\, hi-fi \hi-fi\[shortening of high fidelity.] n.
   An electronic device that plays phonograph records,
   reproducing the original sound with a high degree of
   fidelity. It superseded the older phonographs, and itself is
   being displaced in popularity by CD players.

   Syn: hi-fi, high fidelity sound system.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

hi-fi
    adj 1: characterized by minimal distortion in sound
           reproduction; "a high-fidelity recording"; "a hi-fi
           system" [syn: high-fidelity, hi-fi]
    n 1: equipment for the reproduction of sound with high fidelity
         [syn: hi-fi, high fidelity sound system]
    2: the reproduction of sound with little or no distortion [syn:
       high fidelity, hi-fi]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

HIFI
       Hypertext Interface For Information (ESPRIT)