"In our day, Squibs were often hushed up..."
-- Aunt Muriel (DH8)
A Squib is 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).
There were "Squib-Rights" marches in the late 1960s, when Eugenia Jenkins was Minister for Magic (Pm).
- Argus Filch (CS9)
- Arabella Figg (OP2)
- Dolores Umbridge's brother (Pm)
- Marius Black (BFT)
- Angus Buchanan, professional Rugby player, author of My Life as a Squib
- The seven sons of Thaddeus Thurkell (FW)
- Martha Steward, who lived as a No-Maj (Pm)
The word "Squib" is often used as a prejudicial insult, denoting someone with a disappointing lack of magical power or beneath contempt.
- Wizard Thaddeus Thurkell (1632 – 1692) had seven sons who all turned out to be Squibs. He was so appalled by his non-magical children, he transfigured them into hedgehogs (FW).
- The Black Family had a long tradition of blasting the names of Squibs off their family tapestry (JKR, BFT).
- Scottish Rugby player Angus Buchanan was cast out of his home by his father after the Hogwarts Sorting Hat declared he was non-magical (Pm). He was the only Squib who ever reached the Sorting Ceremony.
- In a Pensieve memory, Marvolo Gaunt referred to his daughter Merope as "disgusting" and a "dirty little Squib" although she was simply a witch with low self-esteem (HBP10).
- Dolores Umbridge pretended her Squib brother did not exist (Pm).
- Neville Longbottom's family believed he was non-magical for many years, leading him to refer to himself as "almost a Squib" even though he was a Pureblood (CS11).
- Hagrid called Filch a "sneakin' Squib" even though he himself was often derided for being a Half-Giant (HBP19).
- Ron's Aunt Muriel assumed that Dumbledore's sister Ariana was a Squib because she was kept hidden away, when actually she was kept away from others because she had dangerous powers (DH8).
- Gellert Grindelwald (disguised as Auror Percival Graves) insulted Credence Barebone by saying he was not a real wizard but "smelled" like a Squib, which proved an almost fatal mistake (WFT). Credence had merely suppressed his magic until a powerful Obscurus formed inside him, which suddenly came roaring out in a burst of Dark energy as he attempted to kill Graves/Grindelwald.
- Harry's son Albus Potter sometimes had trouble with magic, leading Karl Jenkins to call him "Albus Potter, the Slytherin Squib" (CC1.4).
"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
It is unclear whether Squibs are considered Underbeings. When the term is introduced in CG, it is defined as "humanoids" who have "magical ancestry" but no magical powers. This seems like it could include Squibs, but the individuals deemed Underbeings in the stage directions have horns or otherwise do not quite resemble the typical human being. -BB
From the Web
Everything you need to know about Squibs from Wizarding World (originally on Pottermore)
HP, genetics and squibs by temaris
Squib on Harry Potter Wiki
Squibs within the Wizarding World by Starkiller