I'm no philosopher, but I had to something to do with definitions in my life.
No, words of natural languages are not ultimately defined by one word. Try going on a trip around your dictionary. Take an entry, take one word from it. Look it up, then take another word from its definition. Look it up and so on. You will see that you can really go around sometimes, by which I mean that you can eventually find yourself back where you started. There are some philosophical approaches that prefer to use some undefined words though--the way you wanted it.
Unfortunately, I have little idea about the way lawyers deal with this. I only know that they do it very strangely indeed sometimes, at least in my country.