Of basic importance; essential: "Virtually all cultures around the world use the word heart to describe anything that is core, central, or foundational" (Robert A. Emmons).
core (n.) early 14c., "heart or inmost part of anything" (especially an apple, pear, etc.), of uncertain origin, probably from Old French cor, coeur "core of fruit, heart of lettuce," literally "heart," from Latin cor "heart," from PIE root *kerd- "heart."