Snape was a Muggle from what it said in the Prophet.'
'Yeah, that fits,' said Harry. 'He'd play up the pure-blood side so he could get in with Lucius Malfoy and the rest of them.'
Half-Blood Prince Chapter 30: "The White Tomb"

(Quote courtesy of DavidS' Answer)

A:

As I recall, his father is a muggle, and his mother a witch.  This would qualify him as half-blood under the blood purity classification.  Additionally, his mother's last name is Prince.  He would go by Half-Blood Prince because he values his mother's lineage over his fathers, possibly because if he claimed his blood status openly, it would be harder to use it against him, and perhaps because the whole sounds more mysterious or interesting.

from the general population survey Life & Health---young people conducted in year 2014 among Swedish adolescents in Örebro County. The self-administered paper and pencil survey was distributed to all adolescents in grade seven and nine in compulsory school, and in the second year of upper secondary school. The purpose of the survey was to investigate adolescents living conditions, health-related behaviours and health, results from the survey are described elsewhere \[[@CR30]\]. The survey was distributed in two versions, one to adolescents in grade seven and one to adolescents in grade nine and in the second year of upper secondary school. Whereas a majority of questions were common between surveys, questions regarding e.g. sexual behaviours and illicit drugs were only asked to adolescents in school year nine and in the