The terms nap and pile are often used interchangeably to indicate any fabric with a fuzzy surface. The term napped fabric is almost always used to describe anything that changes color depending on how the fuzz lies. Sewing patterns will refer to the “with nap” layout when giving directions on how to cut the pattern.
But nap and pile are not the exact same thing. For the sake of accuracy, here’s the difference.
A napped fabric is woven in the regular manner, and the surface of that fabric is then brushed or teased to raise the surface of the fibers up to some degree. Flannel and fleece are napped fabrics, although it’s rare that the nap is so pronounced as to have the fabric changing color when brushed in particular direction.
Piled fabrics are woven with extra yarns, those yarns being made into lots (and lots) of little loops. Those loops might be cut open, to create velvet, velveteen, velour and corduroy. Or the loops can be left intact, leaving you with terrycloth.
To get really particular, piled fabrics are a sub-type of napped fabrics, which is why you’ll sometimes see the term piled nap when describing velvet, corduroy, etc.
In the everyday sewing world, the term ‘napped fabric’ is used to describe everything directionally fuzzy, but now you know the difference, which might come in hand when you’re on the final round of Jeopardy.