The word "base" refers to the bottom or fundamental aspect of something. Some synonyms for "base" include foundation, core, underlying, root, basis, groundwork, and footing. "Foundation" and "groundwork" both specifically refer to the structure or groundwork upon which something is built or established. "Core" and "underlying" indicate the deeper, more essential aspect or idea behind something. "Root" also implies that the base is the origin or starting point. "Footing" emphasizes stability and support, suggesting a solid base to stand on. Regardless of which synonym is used, they all convey the idea of something being built upon or established from the bottom up.