In Roman mythology, the water nymph Egeria ("of the black poplar") was associated with Diana, presiding over childbirth (for her aid was invoked by women in labor), and sharing her wisdom and prophecy. Egeria was one of the Camenae who were superceded by the Muses as Rome fell under the cultural hegemony of Greece; so Dionysius of Halicarnassus listed her among the Muses (ii. 6o). Egeria seems to predate Roman myth and to have been of Etruscan origins, for she was a nymph consort to Numa Pompilius, second king of Rome, whom she would meet in her sacred grove and whom she taught matters relating to being a wise and just king (Livy i. 19; Juvenal iii. 12).
When Numa Pompilius died, she changed into a well (Ovid, Metamorphoses xv. 479), either the one close by Rome, at Porta Capena, or located in the sacred forest of Aricia in Latium, the grove of Diana Nemorensis ("Diana of Nemi"). "At Aricia there was also a Manius Egerius, a male counterpart of Egeria" (Encyclopędia Britannica 1911).
The name is used as an eponym for a woman advisor or counselor.
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