It comes from the word definately. So So Def Recordings, Def Leopard, Def Jam Recordings also has def in their name. Def Leppard's name comes from an intentional misspelling of the words Deaf Leopard.
Mia Farrow is the girlfriend of Mos Def.
Mos Def was born on December 11, 1973.
It is short for "Most Definitely", which is also what he uses as his website e-mail address. Hence; Mos Def....
Mos Def does produce (usually in collaboration), but hasn't worked on production for anyone besides himself, with the exception of America by K'naan.