base

Something's base is its lower part; the solid part that touches the floor or touches the earth and keeps the other parts of the thing up.

Something's base is its most important part.

A base is a building for military personnel and material.

An base is a chemical that releases hydroxide ions in water, and has the pH of over 7.

Example: Ammonia is a weak base.

Rude, unfair, or unkind.

Example: The people who make those sex videos are really base.