краеугольный

From LSJ

κρατίστην εἶναι δημοκρατίαν τὴν μήτε πλουσίους ἄγαν μήτε πένητας ἔχουσαν πολίτας → the best democracy is that in which the citizens are neither very rich nor very poor (Thales/Plutarch)

Source

Russian > Greek

ἀκρογωνιαῖος, θεμέλιος