Snape's special title

[Gomes Moor]

No: HPFGUIDX 56062

Hi!

I'm delurking for a minute just to ask you: does anyone have any idea 
about why Snape is the only member of Hogwarts' staff who is 
called "master"?

There is a passage at GoF where this is quite remarkable:

"Harry scanned the table more carefully. Tiny little Professor 
Flitwick, the Charms teacher, was sitting on a large pile of cushions beside Professor Sprout, the Herbology teacher, whose hat was askew over her flyaway gray hair. 
She was talking to Professor Sinistra of the Astronomy department. On 
Professor Sinistra's other side was the sallow-faced, hook-nosed, greasy-haired Potions master, Snape." (GoF, US, ch 12)

Gomes Moor