For fabric what is nap?

'Nap' is a term used for fabric that has a 'pile' such as velvet, corduroy, and suede. In other words not smooth, but with a fuzzy, fur-like surface. When you sweep your hand across the fabric, the threads will stand up in one direction, and lay down in the other direction.

When using fabric like this, you want all pattern pieces to have the 'nap' going the same direction, so you have to use extra care in laying them out. Usually you want the nap to lay down from the top of the garment towards the bottom.