A small object which is thought to bring good luck.
Lockhart claimed to have dealt with a case of Transmogrifian Torture in Ouagadougou by giving the townsfolk protective amulets (CS9). In 1992, when the students realised that a monster was attacking Muggle-borns, a booming trade in magical remedies and defences was carried on in secret, including protective amulets. Most if not all of the items for sale were fake (CS11). In 1996, a wizard in Diagon Alley attempts to sell Mrs Weasley an amulet, which according to his signage is 'effective against Werewolves, Dementors and Inferi'. (HBP6)
The word "amulet" comes from the Latin amulētum; the earliest extant use of the term is in Pliny's Natural History, meaning "an object that protects a person from trouble". (https://en.m.wikipedia.org/wiki/Amulet)