A brick of ice cream with multiple flavors in it, usually chocolate, vanilla, and strawberry. Of course, too many people who buy Neapolitan ice cream in the grocery store only eat one flavor and leave the other two to freeze into a solid, inert mass. More sensible people eat every possible combination of the three flavors, 'cause seriously, chocolate, vanilla, and strawberry all taste fantastic together. 

Oh, and a Neapolitan is also a native of Naples, Italy. You might have expected that Neapolitan ice cream was invented by a Neapolitan, but it appears it was first created by a Prussian chef in the late 1830s. It used to be that the combined flavors were pistachio, vanilla, and cherry, in an approximation of the Italian national flag. But chocolate, vanilla, and strawberry flavors were much more popular, so that's now the standard.