Not an exhaustive definition, but true to the core meaning of the word:
-serving as, or being an essential part of, a foundation or basis; basic; underlying: fundamental principles; the fundamental structure.
-of, pertaining to, or affecting the foundation or basis: a fundamental revision.
-being an original or primary source: a fundamental idea.
-a basic principle, rule, law, or the like, that serves as the groundwork of a system; essential part: to master the fundamentals of a trade.
-strict adherence to any set of basic ideas or principles
The current connotation to this word applied to beliefs is actually, in many ways, contrary to the actual word itself. Those referred to as fundamentalists or "fundies" are often not fundamental in their beliefs, but actually extremists when compared to those who follow a fundamental value set within the same general belief system. I realize that the current use has grown from the use of the word referring to the fundamentalist movement, but too often the ones that seem to be getting the press are more extremist than fundamentalist in any sense of the word.
This is just something that bugs me when I see the word used this way all the time.