The Collaborative International Dictionary of English v.0.48:

Cachinnation \Cach`in*na"tion\ (k[a^]k`[i^]n*n[=a]"sh[u^]n), n.
   [L. cachinnatio, fr. cachinnare to laugh aloud, cf. Gr.
   kacha`zein.]
   Loud or immoderate laughter; -- often a symptom of hysterical
   or maniacal affections.
   [1913 Webster]

         Hideous grimaces . . . attended this unusual
         cachinnation.                            --Sir W.
                                                  Scott.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cachinnation
    n 1: loud convulsive laughter