"There's a loophole in the law, you'll find .... As long as he wasn't intending to fly the car, the fact that the car could fly wouldn't -"
"Arthur Weasley, you made sure there was a loophole when you wrote that law! Just so you could carry on tinkering with all that Muggle rubbish in your shed! And for your information, Harry arrived this morning in the car you weren't intending to fly!"
-- Arthur and Molly Weasley's discussion of illegal charmed objects (CS3)
Muggle objects which are specifically not allowed to have magical spells applied to them.
The Muggle artifacts are listed in the Registry of Proscribed Charmable Objects. As these are objects that would normally be used by Muggles in their daily lives, there is the risk that an enchanted object might fall into the hands of the Muggles.
"Artefact" is the British spelling of the noun meaning, primarily, an object shaped by human workmanship, especially one of historical or archaeological interest. "Artifact" is the American spelling.