Fundamental (adjective)

1. Forming a necessary base or core; of central importance.
2. Basic and essential, serving as the foundation for something.
3. Relating to or affecting the foundation or basis of something.
4. Relating to or dealing with the most basic and important aspects of something.

Origin:

From middle english fondamentale, from old french fondamental, from medieval latin fundamentalis, from latin fundamentum ("foundation"), from fundare ("to lay the foundation for, establish, create").

Examples:

1. The fundamental laws of physics.
2. This is a fundamental problem.
3. A fundamental change in policy.
4. Fundamental human rights.
5. A fundamental principle of democracy.