Trace

The Trace is a charm put on witches and wizards who are under seventeen years of age. When any magical activity is performed in the vicinity of the under-age wizard (or witch) the Ministry of Magic and Improper Use of Magic Office is alerted to the spell that was used and the location of the offending wizard. On the seventeenth birthday of the wizard the Trace is broken and the wizard is no longer under the Decree for the Reasonable Restriction of Underage Sorcery.

Remus Lupin makes note that it is impossible for the Trace to continue to work after a wizard has had his seventeenth birthday.

It is unknown how the witch or wizard is imbued with the Trace, but it is known to have faults. Ministry officials trust parents to properly punish children if they perform magic, due to the fact that the parents' own magic will constantly interfere with the Trace. In the case of Harry Potter before he turned seventeen any magic around him would prompt the Trace to alert the Ministry, since he is the only magical person living in his neighborhood and thus would be the only one capable of performing the magic (or so the Ministry believes).

Uses

 * It is used extensively throughout the series, but is not explicitly named until Alastor Moody makes mention of it in 1997.

Name origin
Trace seems to simply describe how magic around the under-age witch or wizard is "traced" to her or his location.

Appearances

 * Entire series; mentioned in Harry Potter and the Deathly Hallows.