Concealment Charms (incantation unknown) are spells used to conceal things from view and to obfuscate inherent magical properties of objects.
In the 15th century, evil wizard Ekrizdis placed concealment charms onto his personal fortress Azkaban to conduct his Muggle tortures in privacy. After his death, the charms faded away, allowing authorities to acknowledge the island's existence for the first time.
In 1793 or 1794, Vincent Duc de Trefle-Picques, a French wizard and an alumnus of Beauxbatons Academy of Magic, performed a concealment charm on his own neck and pretended his head had already been severed to escape the Reign of Terror following the French Revolution.
Sometime in the early 20th century, Minister for Magic Ottaline Gambol allowed a secret operation to obtain a train to be used as a means of transport of Hogwarts School students to the Castle and back, on the beginning and end of term, respectively. This operation ended up involving one hundred and sixty-seven Memory Charms and the largest ever mass Concealment Charm performed in Britain, while several bemused Muggle railway workers in Crewe spent the rest of the year grappling with the uncomfortable feeling that they had mislaid something important.
In 1996, when Harry Potter speculated that Draco Malfoy had sneaked a cursed opal necklace by Filch's Secrecy Sensor, Hermione Granger said that Secrecy Sensors picked up concealment charms easily.