Fundamental
Part of speech: adjective
Definition: Pertaining to the foundation or basis; serving for the foundation. Hence: Essential, as an element, principle, or law; important; original; elementary; as, a fundamental truth; a fundamental axiom.
Part of speech: noun
Definition: A leading or primary principle, rule, law, or article, which serves as the groundwork of a system; essential part, as, the fundamentals of linear algebra.
Example sentence: In my country of South Africa, we struggled for years against the evil system of apartheid that divided human beings, children of the same God, by racial classification and then denied many of them fundamental human rights.
Important
Part of speech: adjective
Definition: Having relevant and crucial value.
Example sentence: Family is not an important thing. It's everything.