The Collaborative International Dictionary of English v.0.48:

Sporadic \Spo*rad"ic\ (-r[a^]d"[i^]k), a. [Gr. ? scattered, fr.
   ?, ?, scattered, fr. ? to sow seed, to scatter like seed: cf.
   F. sporadique. See Spore.]
   Occurring singly, or apart from other things of the same
   kind, or in scattered instances; separate; single; as, a
   sporadic fireball; a sporadic case of disease; a sporadic
   example of a flower.
   [1913 Webster]

   Sporadic disease (Med.), a disease which occurs in single
      and scattered cases. See the Note under Endemic, a.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sporadic
    adj 1: recurring in scattered and irregular or unpredictable
           instances; "a city subjected to sporadic bombing raids"
           [ant: continual]