something that produces an effect, that makes something happen

agent, originator

a stone forming a corner of a foundation for a building

vital element

an apparatus producing a rush of water into the air; a source or origin

source, often of liquid

a bar passed through the end of an axle to keep the wheel in position; a person who keeps a business, organization etc successful

essence, theme

a cause or reason for doing something

reason, purpose

a place or reason from which something starts

cause, basis

the part of a plant, tree etc in the ground and which takes water and food from the soil

base, core