Meaning:
* Priori: Latin for "prior" or "before"
* Incantatem: Latin for "incantation" or "spell"
The spell's function:
When two wands share a core, and one wand kills the other's owner, the surviving wand will "remember" the last spell cast by the dead wizard. This spell is forced from the surviving wand, often in the form of ghostly replicas of the spells cast.
In the Goblet of Fire:
The spell is crucial to the climax of the book. When Cedric Diggory is killed by Peter Pettigrew, Harry's wand (which shares a core with Voldemort's) forces the spell "Avada Kedavra" (the Killing Curse) out of its tip, creating a ghostly image of Cedric and Voldemort. This revelation proves that Voldemort killed Cedric and that Harry was not the murderer.
Key points about Priori Incantatem:
* It's a very powerful spell, but not a "normal" spell in the sense that you can choose to cast it. It's a reaction to specific conditions.
* It only works when two wands share a core (like Harry's and Voldemort's).
* It's not just about the last spell, but rather about the "last act of magic" performed by the wand.
Importance to the story:
The spell helps to clear Harry's name and confirm Voldemort's return. It also foreshadows the close connection between Harry and Voldemort, as well as the powerful connection between their wands.