The Trace is the charm that detects magical ability around under-seventeens.
References from the canon
- Moody said that if Harry or anyone around him cast a spell to get him out of number four, Privet Drive, Pius Thicknesse and the Death Eaters would know about it thanks to the Trace (DH4).
- Discussed by Harry and Ron as the principal reason why they had to wait until after Harry's seventeenth birthday to begin hunting for Horcruxes (DH6). When the time came, Harry initially revelled in its removal (DH7).
- Ron said that it's wizarding law that the Trace breaks at seventeen and that it cannot be put on an adult (DH9). When asked if Harry's Trace could still be on him, Remus Lupin said 'Impossible', then added that for one thing, if it had been the Death Eaters would have known for certain where Harry was (DH11).