A non-magical person born to at least one magical parent (Pm). They are much rarer than Muggle-born witches and wizards (CS9).
A Squib is not a Muggle. Born to a wizarding family, a Squib has such a low level of magical power that he or she is essentially unable to do any magic at all. However, while a Squib cannot cast spells, he or she can apparently see magical beings such as poltergeists, though not dementors (JKR).
"squib" - Eng. a dud firework that won't ignite properly
Some Squibs seem to have formed special bonds with cats, whom they refer to as Mr or Mrs. It is possible that these cats function as guides and aides to Squibs as they live in a world in which they don't really fit. In a sense, these cats may be the wizarding equivalent of guide dogs and other animals which are trained to help Muggles with disabilities.
Indeed, discussants on Episode 4 of the Mugglenet Academia Podcast discussed squibs and whether they might be considered as living with a disability (i.e. not able to perform magic) in the eyes of the Ministry of Magic and Wizarding law. Analogies were made to the Americans with Disabilities Act, The Equal Protection under the Law Cause, as well as similar legislation in British Common Law. --Hufflepuffskein