┖ Harry Potter and The Half-Blood Prince - Chapter 23 “Horcrux”
"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 the term. A Horcrux is the word used for an object in which a person has concealed part of their soul."
"I don’t quite understand how that works, though, sir," said Riddle.
His voice was carefully controlled, but Harry could sense his excitement.
"Well, you split your soul, you see," said Slughorn, "and hide part of it in an object outside the body. Then, even if one’s body is attacked or destroyed, one cannot die, for part of the soul remains earthbound and undamaged. But of course, existence in such a form …"