foundation
the base something rests on
Root
- fund / fundus: bottom, base, foundation, or depth
Other words with the fund / fundus root
- fundamental: basic, central, or forming the base
- profound: deep in meaning, feeling, or understanding
- profundity: deep insight or depth of meaning
Foundation is the familiar base: what a building or argument rests on.
The first theorem became the foundation for the rest of the proof.