flounce 1 (flouns)
A strip of decorative, usually gathered or pleated material attached by one edge, as on a garment or curtain.
(from The Free Dictionary)
This is the traditional definition of flounce.
However, in modern couture, a flounce is a circular piece of fabric that is straightened to create a ruffled decoration, while a ruffle is a straight piece of fabric which has been gathered and attached to an edge as a decoration.
So, a flounce starts with a circular or oval piece of fabric, and a ruffle starts with a straight piece of fabric.
The advantage of a flounce is that there is no need to ease or gather the edge. This does not add bulk to the seam.