A Nithling is a broad term for any of a wide varety of creatures, generally dangerous, that are formed directly from Nothing in Garth Nix's Keys to the Kingdom series of books. Nithlings can be summoned forth by entities with sufficient capability to manipulate Nothing, but a great many Nithlings form of their own accord when large enough amounts of Nothing make their way from the Void of Nothing into the House or, very rarely, the Secondary Realms.
There are a great many varieties of Nithling. One of those most commonly seen in the books is the Fetcher, which resembles a dog-faced man in a smart suit and bowler hat. Another type of Nithling which is used as a major plot element is the Cocigrue (the individual one in question being known as the Skinless Boy), which can imitate the appearance of a mortal from the Secondary Realms or a Denizen of the House. Like most entities from the House (including its resident Denizens), they have an adverse effect on the Secondary Realms merely by entering them, such as the spontaneous generation of highly dangerous new viruses.
Nithlings can generally be repelled by either salt or silver, and some (particularly Fetchers) cannot cross thresholds uninvited, although the more powerful the Nithling is, the less effective these defensive measures are. Nithlings can be destroyed instantly merely on contact with one of the Keys to the Kingdom, and sorcerously formed ones like the Cocigrue can be banished by plunging the item which was used in their creation into Nothing.
Later in the series, the New Nithlings, or NewNiths, are introduced. Created by the Architect's son, the Piper, they are very close to the Architect's own Denizens. Unlike normal Nithlings, they do not have destructive instincts, and would in fact rather be farmers. They are Nithlings only by virtue of having been made from Nothing (although that is ultimately true of all things), and they are perhaps better called, as the Piper himself names them, New Denizens.