Muggles' Guide to Harry Potter/Magic/Nox

Overview
Nox is the counter-spell to Lumos; it darkens a lit wand.

Extended Description
The Nox spell is used by Harry and Hermione in Harry Potter and the Prisoner of Azkaban, and in the depths of Gringotts in Harry Potter and the Deathly Hallows, to extinguish their wands, previously illuminated by means of the Lumos charm, when they no longer needed their light.

Analysis
An extinguishing spell is, in retrospect, necessary; otherwise, you have the equally ludicrous cases of a wand that, once illuminated, never goes out, or of a wand that, once lit, goes out eventually of its own accord, likely at the worst possible moment. It is surprising, perhaps, that we see the extinguishing charm used so very infrequently, given the number of times that wandlight is invoked. However, looking at the behavior of magic in the stories, this is something of a pattern; time-limited spells, such as the effects of Polyjuice Potion only wear off inconveniently when such is necessary to the story line, and otherwise remain in force as long as necessary for the action, but no longer.