If a Muggle was born of a wizard/witch and a Muggle, is that person a Squib, or is that reserved to people born from pure-blood families?
(Related: Is a Squib a Muggle?)
If a wizard and Muggle have a nonmagical child, it’s a Squib. In the Pottermore writing on Dolores Umbridge, it’s mentioned that her parents (a wizard father and Muggle mother) have two children together - Dolores, a witch, and her brother, who was a Squib.
Dolores Jane Umbridge was the eldest child and only daughter of Orford Umbridge, a wizard, and Ellen Cracknell, a Muggle, who also had a Squib son.
- Dolores Umbridge (Pottermore)
This shows that Squibs can be born to a wizard/Muggle couple - in that case the nonmagical child would still be considered a Squib, not a Muggle, despite having one Muggle parent. It also shows by extension that Squibs don’t have to be pure-blood, as any child with a Muggle parent isn’t.
No I don't think so. Kendra Dumbledore was definitely a Muggle-born witch:
'Dumbledore's mother was a terrifying woman, simply terrifying. Muggle-born, though I heard she pretended otherwise -'
'She never pretended anything of the sort! Kendra was a fine woman,' whispered Doge miserably, but Auntie Muriel ignored him.
And yet nobody has any problem speculating that Arianna was a Squib:
'- proud and very domineering, the sort of witch who would have been mortified to produce a Squib -'
Harry Potter and the Deathly Hallows - p.129 - Bloomsbury - Chapter 8, The Wedding
A Muggle wouldn't be born of a wizard/witch and Muggle, instead a Squib would be born. That's because since the child contains traits of a Muggle and a magical person, he/she wouldn't be purely non-magical, and he/she would have some magical characteristics inherited from the magical parent. Hence the child would become a Half-Blood Squib...