I'm sure you've heard the expression "elegant dress" - usually a neat, tailored dress or suit suggestive of sophistication and class. However, such a dress, no matter how classy, won't look elegant on someone with a punk hairstlye, neon lipstick, and who walks in a slouched manner. Although elegance does have a lot to do with the choice of clothing and hairstyle, generally speaking elegance is having an air of sophistication and class, of good taste and feminine grace. It is how a lady walks, stands, sits, acts, and generally carries herself. It conjures up images of actresses from the golden age of Hollywood such as Ingrid Bergman, Grace Kelly, Greta Garbo, Lana Turner, Audrey Hepburn, Katherine Hepburn and so on. Not all beautiful, and certainly not all "sexy", but they all had an air about them that "elegant" describes more than any other word.
Of all the factors here, this one is the easiest to achieve, because to a large extent it can be learned and applied, although it is also probably the hardest to carry off if not learned from an early age. Furthermore it seems to be on the decline, perhaps because the sexual revolution has changed the idea that a "lady is a lady" into an equality that has no time or place for the subtle airs from a bygone era. Nevertheless, in my opinion it is as important today as any of the other factors presented here on my site, and to a large extent goes hand-in-hand with femininity.