“Well,” said Slughorn, not looking at Riddle, but fiddling with the ribbon on top of his box of crystallized pineapple, “well, it can't hurt to give you an overview, of course. Just so that you understand t he term. A Horcrux is the word used for an object in which a person has concealed part of their soul.”

HP&HBP, part 23.

“It should be, actually! But my point is that whatever happens to your body, your soul will survive, untouched,” said Hermione. “But it's the other way round with a Horcrux. The fragment of soul inside it depends on its container, its enchanted body, for survival. It can't exist without it.”

HP&DH, part 6.

“Yes,” said Hermione, now turning the fragile pages as if examining rotting entrails, “because it warns Dark wizards how strong they have to make the enchantments on them. From all that I’ve read, what Harry did to Riddle’s diary was one of the few really foolproof ways of destroying a Horcrux.”

HP&DH, part 6.