The preserved remains of a body, human or animal, from which fluids have been removed.
Although mummification can occur through natural processes in very dry conditions, the most common conception of a mummy is that of one deliberately embalmed as a preparation for burial, for which additional preservative measures have been performed. In ancient Egypt, mummification was performed on the bodies of humans and of cats, and was considered a necessary step in preparing the deceased for the afterlife.
It is unclear whether the type of mummy Parvati Patil fears (which the boggart impersonated during her first Defence Against the Dark Arts lesson with Remus Lupin, PA7), is an actual mummy animated or some Dark Creature resembling a mummy. If the latter is the case, a mummy is a frightening creature, bandaged, bloody, and sightless, possibly a manifestation of a curse left behind by ancient wizards (PA7). (Of course, if Parvati was exposed to Muggle entertainment as a child, she may just have watched too many horror movies.)