The Collaborative International Dictionary of English v.0.48:

Jagannath \Jag"an*nath\, Jagannatha \Jag`an*na"tha\, Jaganatha
\Jag`a*nat"ha\,, prop. n. Also Juggernaut \Jug"ger*naut\ [Hind.
   Jagan-n[=a]th lord of the world, Skr. jagann[=a]tha.]
   (Hinduism)
   A particular form of Vishnu, or of Krishna, whose chief idol
   and worship are at Puri, in Orissa. The idol is considered to
   contain the bones of Krishna and to possess a soul. The
   principal festivals are the Snanayatra, when the idol is
   bathed, and the Rathayatra, when the image is drawn upon a
   car adorned with obscene paintings. Formerly it was
   erroneously supposed that devotees allowed themselves to be
   crushed beneath the wheels of this car. It is now known that
   any death within the temple of Jagannath is considered to
   render the place unclean, and any spilling of blood in the
   presence of the idol is a pollution.
   [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Juggernaut \Jug"ger*naut`\, n. [Skr. jagann[=a]tha lord of the
   world.]
   1. One of the names under which Vishnu, in his incarnation as
      Krishna, is worshiped by the Hindus. See also Jagannath.
      [Written also Juggernnath, Jaganath, Jagannath,
      Jaganatha, Jagannatha, etc.]
      [1913 Webster]

   Note: The principal seat of the worship of Juggernaut
         (Jagannath) is at P[^u]ri in Orissa. At certain times
         the idol is drawn from the temple by the multitude, on
         a high car with sixteen wheels. The idol is considered
         to contain the bones of Krishna and to possess a soul.
         The principal festivals are the Snanayatra, when the
         idol is bathed, and the Rathayatra, when the image is
         drawn upon a car adorned with obscene paintings.
         Formerly it was erroneously supposed that fanatical
         devotees threw themselves under the wheels of this car,
         to be crushed as a sacrifice to the god. It is now
         known that any death within the temple of Jagannath is
         considered to render the place unclean, and any
         spilling of blood in the presence of the idol is a
         pollution. As a result of this erroneous belief,
         however, the word juggernaut is now used principally
         in the figurative sense 2.
         [1913 Webster +PJC]

   2. Any large, unstoppable force, power, or popular movement
      which defeats or destroys any person who gets in its way
      or attempts to stop it; as, for years the Notre Dame
      football team was an unstoppable juggernaut; after the
      early primaries, Johnson's campaign became a juggernaut,
      crushing all rivals.
      [PJC]

WordNet (r) 3.0 (2006):

juggernaut
    n 1: a massive inexorable force that seems to crush everything
         in its way [syn: juggernaut, steamroller]
    2: an avatar of Vishnu [syn: Jagannath, Jagannatha,
       Jagganath, Juggernaut]
    3: a crude idol of Krishna