Question about Half-bloods. No spoilers.

[Carey McGinley]

No: HPFGUIDX 62465

Hi, this doesn't really have spoilers in it, but
something I've noticed is that I believe in book four
and again in the Order of the Phoenix, they mention
Harry being a half-blood like Voldemort.  My question
is this.  Isn't a half-blood a wizard or witch who's
born from a married wizard or witch to a muggle? 
Seamus says in the first book "My dad's a muggle, me
mum's a witch, bit of a nasty shock for him when he
found out."  And Voldemort was born to Riddle, a
muggle and his mother.  Which makes me curious as to
why they say Harry's a half-blood when both of his
parents are of magic blood?
Is a half-blood determined when your parents' family
isn't magical, because that I'd understand, Lily was
muggle born, and James was born of magic, I believe
anyway.
I was just wondering if anybody knew what the real
definition of a half-blood is, since Harry is one. 
Sorry to bother everybody, thanks in advance. 
~*Carey*~     

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