The Collaborative International Dictionary of English v.0.48:

Informant \In*form"ant\, n. [L. informans, -antis, p. pr. of
   informare. See Inform, v. t.]
   1. One who, or that which, informs, animates, or vivifies.
      [Obs.] --Glanvill.
      [1913 Webster]

   2. One who imparts information or instruction.
      [1913 Webster]

   3. One who offers an accusation; an informer. See Informer.
      [Obs. or R.]
      [1913 Webster]

            It was the last evidence of the kind; the informant
            was hanged.                           --Burke.
      [1913 Webster]

WordNet (r) 3.0 (2006):

informant
    n 1: a person who supplies information [syn: informant,
         source]
    2: someone who sees an event and reports what happened [syn:
       witness, witnesser, informant]