Real Dictionary

fundamental
You use fundamental to describe something which exists at a deep and basic level, and is therefore likely to continue.
You use fundamental to describe things, activities, and principles that are very important or essential. They affect the basic nature of other things or are the most important element upon which other things depend.
adjective: If one thing is fundamental to another, it is absolutely necessary to it, and the second thing cannot exist, succeed, or be imagined without it.

fundamental in American English
of or forming a foundation or basis; basic; essential
adjective: serving as, or being an essential part of, a foundation or basis; basic; underlying
noun: a basic principle, rule, law, or the like, that serves as the groundwork of a system; essential part

fundamental in British English
adjective: of, involving, or comprising a foundation; basic
noun: a principle, law, etc, that serves as the basis of an idea or system