"Nymphet" is Humbert's word for an attractive young girl, but he goes to great lengths to define it precisely, particularly the age range and the exact physical qualities a "girl-child" (what we might today call a "tween") must have in order to qualify for nymphet status:
Between the age limit of nine and fourteen there occurs maidens who, to certain bewitched travelers, twice or many times older than they, reveal their true nature which is not human but nymphic (that is demonaic); and these chosen creatures I propose to designate as "nymphets." (1.5.6)
Now, whether or not anyone would agree to call these girls "nymphets" remains unknown. This definition is Humbert's own, for Humbert's pleasure, and to clarify for the reader ("the jury") and his lawyer, who prompted him to write the account, exactly what constitutes the object of his lust.
Because we're dealing with a verbal trickster, it's important to pause and consider Humbert's definition. Words like "bewitched" echo his whole Enchanted Hunters theme of magic, casting spells, and fairy lands, but it also implies that the man who loves the nymphet almost cannot help himself because he is in her power, she is the one casting as spell and is thus in control – as Humbert suggests of Lolita when he tells us that she seduced him. After his initial more physical description of "nymphet," Humbert makes a further point:
What drives me insane is the twofold nature of this nymphet—of every nymphet […] of tender dreamy childishness and a kind of eerie vulgarity. (1.11.13)
Not only does Humbert delineate "nymphet," but he also explains that one who loves nymphets must be an "artist and a madman, a creature of infinite melancholy" (1.5.6) – like him.