i dont think you can 'define' words per se. you can only teach a person how to you use it, and as they you use it more and more, in different environments, etc, they will get better at using it.
like as a few comedy tips, i would not use the word 'person' about a horse, although i might use it about a pet dog. i would obviously use it about people (coma or not, lol) but not about foetuses..hope that helps your search.
(ps: Singer is a utilitarian. which to me is like the moral equivelant of totalitarian democracy - uncool).
in conclusion, the meaning of a word is not in some magical definition, it is in your straightforward use of the word. this means that all you need do is ask a few practical questions about its use, and these questions will no doubt differ, depending on the word in question.
im sure everyone will hate me for this, lol.
peas n glove.