Now I wish to introduce the following idea. Between the age limits of nine and fourteen there occur maidens who, to certain travelers, twice or many times older than they, reveal their true nature which is not human, but nymphic (that is, demoniac); and these chosen creatures I propose to designate as "nymphets."
It will be marked that I substitute time terms for spatial ones. In fact, I would have the reader see "nine" and "fourteen" as the boundaries - the mirrory beaches and rosy rocks - of an enchanted island haunted by those nymphets of mine and surrounded by a vast, misty sea.
For Humbert, a nymphet is in the earliest stages of puberty - "The bud-stage of breast development appears early (10.7 years)". When he meets a streetwalker of 18, he considers her no longer a nymphet, although her body is still in some ways childlike.
When I was a child and she was a child, my little Annabel was no nymphet to me; I was her equal, a faunlet in my own right, on that same enchanted island of time.
...I met the unblinking dark eyes of two strange and beautiful children, faunlet and nymphet, whom their identical flat dark hair and bloodless cheeks proclaimed siblings if not twins.
Nabokov used the word to describe one who could "discern" nymphets from other girls. In Humbert's own words:
A normal man given a group photograph of school girls or Girl Scouts and asked to point out the comeliest one will not necessarily choose the nymphet among them. You have to be an artist and a madman, a creature of infinite melancholy, with a bubble of hot poison in your loins and a super-voluptuous flame permanently aglow in your subtle spine (oh, how you have to cringe and hide!), in order to discern at once, by ineffable signs - the slightly feline outline of a cheekbone, the slenderness of a downy limb, and other indices which despair and shame and tears of tenderness forbid me to tabulate - the deadly little demon among the wholesome children.