Muggle-born

From Hpwiki

Muggle-born is the term for witches and wizards of Muggle parentage. Fanatical Purebloods think themselves better than Muggle-born wizards, but the books are littered with extremely talented Muggle-borns (e.g. Lily Potter or Hermione Granger) and Half-Bloods (see Harry Potter or Tom Riddle).

It is unclear how Muggle-born children come by their magic, though genetics is a fairly reasonable explanation. In canon, we know of both magical and non-magical siblings, so we cannot estimate the rarity of one or the other happening.

It is likely that the child realises their magic through an unusual or life-saving experience, judging by the experience Harry Potter has had, such as his hair growing back after being shaved, and how Neville Longbottom's grandfather continuously tried to throw him out of the window to prove he wasn't a Squib.