Sylph is a noun that refers to a slender woman or girl. 

Coming from the Latin word sylphi (SILL fee) which refers to a mythic nymph of the woods, our word of the day has been around since the mid 17th century. Example:

When I saw the mysterious Sylph in the woods, I thought I was seeing a mythical spirit rise to the sky. But it turned out Erica was just trying out her new silver leotard.