in Greek religion, the Olympian goddess of agriculture and useful vegetation, protectress of the social order and of marriage, mother of Persephone, from Greek Dēmētēr; the second element generally given as māter (see mother (n.1)); the first element possibly from da, Doric form of Greek "earth" (see Gaia), but Liddell & Scott find this "improbable" and Beekes writes, "there is no indication that [da] means 'earth', although it has also been assumed in the name of Poseidon." The Latin masc. proper name Demetrius means "son of Demeter."

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Definitions of Demeter from WordNet

Demeter (n.)
(Greek mythology) goddess of fertility and protector of marriage in ancient mythology; counterpart of Roman Ceres;