The Collaborative International Dictionary of English v.0.48:

Tantrum \Tan"trum\, n.
   1. A whim; an affected air. [Colloq. and archaic]
      --Thackeray.
      [1913 Webster]

   2. A display of ill-humor, especially a demonstration of rage
      or frustration by shouting or violent physical movements,
      such as the stamping of feet; called also temper
      tantrum. It is usually associated with children, but is
      sometimes seen in adults.
      [PJC]

   3. [fig.] A display of anger expressed by irrationally
      striking out at innocent targets or inanimate objects; as,
      the governor was so insulted by the article, he threw a
      temper tantrum and cancelled the ceremony.
      [PJC]

WordNet (r) 3.0 (2006):

tantrum
    n 1: a display of bad temper; "he had a fit"; "she threw a
         tantrum"; "he made a scene" [syn: fit, tantrum,
         scene, conniption]