The Collaborative International Dictionary of English v.0.48:

Neapolitan ice \Ne`a*pol"i*tan ice\, Neapolitan ice cream
\Neapolitan ice cream\
   (a) An ice or ice cream containing eggs as well as cream.
   (b) An ice or ice cream prepared in layers, as vanilla,
       strawberry, and chocolate ice cream, and orange or lemon
       water ice.
       [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Neapolitan ice cream
    n 1: a block of ice cream with 3 or 4 layers of different colors
         and flavors