Boethius' definition 
The Catholic Encyclopedia refers to Boethius definition of "person" 
"Naturæ rationalis individua substantia (an individual substance of a rational nature)"
as "the classic definition," one on which they elaborate as a basis for further treatment (they get into Trinitarianism, actually). I have to admit its a good definition: person ultimately has something to do with rationality - an individual something of a rational nature. I know you've been doing a lot of good work here Pfhorrest, what do you think? - (t | c)