“But I haven’t got a problem calling him V—”
“NO! ...Sorry...but the name’s been jinxed. Harry, that’s how they track people!"
-- Harry and Ron (DH20)
When put on a word, anyone who then says that word becomes trackable by the caster; saying the word breaks protective enchantments and causes some kind of magical disturbance.
References from the canon
- The Death Eaters placed a Taboo on the word "Voldemort", reasoning that only serious opposition such as the Order of the Phoenix would use it (DH20)
- Harry accidentally broke the magical protections around the campsite where he was staying by slipping and saying the name as he had been used to doing for years, forgetting that it was Taboo (DH22).
- The choice of name for this effect is neat; there was already a social taboo on saying the name before the spell was ever cast.
- Ron refers to the Taboo as the name being jinxed (DH20)
"taboo", Eng. forbidden, particularly when forbidden by social custom, from Polynesian "tabu" meaning "unclean or cursed"