When I was in high school, I took some basic philosophy classes. And my teacher, a thoughtful man from Alabama, taught me one of the most important principles that I still live by to this day:
Always define your terms.
The example that he gave nearly 10 years ago that is still relevant today was of one of the debates regarding abortion. The definition of the term "abortion," he said, was not agreed upon by both sides of the argument; rather, they both held quite different opinions on what that term meant.
In his example, one side contended that an abortion was murder, plain and simple: that's a human being and you're killing it. The other side saw it as a removal of some extraneous body cells - no more murder than washing your hands or plucking your eyebrows.
The result of this broad divide between definitions was that these two sides would never see any sort of agreement or even compromise. Or rather, that they wouldn't as long as they continue to argue about two different concepts that happen to share the same name.
It's a simple example and a simplification of the real abortion debate. But the reasoning stands. If you can't even agree on what it is you're discussing, there's no way the discussion itself will be fruiful.