cornerstone
ˈkɔːnəstəʊn/
noun
noun: cornerstone; plural noun: cornerstones; noun: corner-stone; plural noun: corner-stones
1.
an important quality or feature on which a particular thing depends or is based.
"a national minimum wage remained the cornerstone of policy"
synonyms: foundation, basis, keystone, mainspring, mainstay, linchpin, bedrock, fundament, base, key, fundamental principle, main ingredient, central component, centrepiece, core, heart, centre, focus, crux, prop, backbone, anchor
"the theory of natural selection is a cornerstone of biological thought"