From Longman Dictionary of Contemporary Englishdead languagedead languagea dead language, for example Latin or Ancient Greek, is no longer used by ordinary people → living language at living1(1) → dead
Examples from the Corpus
dead language• Latin is a dead language.• He stared, then said her words himself, the way he'd spoken Latin as a boy. Dead language.• There are some who call Yiddish a dead language, but so was Hebrew called for two thousand years.• Also included is the artificial adoption of dead languages for nationalistic purposes.• He made the dead languages into living literature. 