The Collaborative International Dictionary of English v.0.48:

belly laugh \bel"ly laugh`\, n.
   a vigorous, loud laugh, expressing a strong amusement.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

belly laugh
    n 1: a burst of deep loud hearty laughter [syn: guffaw, belly
         laugh]
    2: a joke that seems extremely funny [syn: belly laugh,
       sidesplitter, howler, thigh-slapper, scream, wow,
       riot]