Petunia

[finwitch]

No: HPFGUIDX 112458

This is an excerpt from http://www.bloomsbury.com/ - the UK 
publisher, latest interview with JKR:

Is Aunt Petunia a Squib?

Good question. No, she is not, but—[Laughter]. No, she is not a 
Squib. She is a Muggle, but—[Laughter]. You will have to read the 
other books. You might have got the impression that there is a little 
bit more to Aunt Petunia than meets the eye, and you will find out 
what it is. She is not a squib, although that is a very good guess. 
Oh, I am giving a lot away here. I am being shockingly indiscreet.

----

Now, I think this means that, an idea that Petunia has as much magic 
as a squib, (enough to see Dementors like Arabella Figg) but not 
enough to be considered a muggle-born witch. Being muggle-born, she'd 
simply be considered a Muggle by wizards...!

Someone came up with a term Mugid for such persons in a fanfic... but 
I doubt wizards in the actual books would ever have come up with that 
term, having no need for it, they'd just call them Muggles...

Finwitch