A Muggle is a person in the Harry Potter continuum who cannot use magic. They are often scoffed at by pureblooded wizards. (According to J.K. Rowling, if a Muggle found a wand and tried to use it, he/she would only succeed in causing an explosion.)
Most PPC agents fall into this category, but ways have been developed to allow nominally Muggle agents to use magic in that particular continuum, even if in a limited fashion (said limited fashion not necessarily involving explosions). Scoffing at them would most likely lead to limbs being removed.