ὄνειρος (oneiros, "OH-nay-ross") is the Greek word for a dream, and the record gives it a middling, tellable life: 114 occurrences across 30 works. The Histories holds the most (23); then the Odyssey (15) and the Iliad (8). After the epics come the tragedies, clustered thickly: Agamemnon (6), Iphigenia in Tauris (6), and Libation Bearers (6). The word belongs to those who wake and must interpret.
The lexicon is single and spare. LSJ (Liddell–Scott–Jones, 9th ed., 1940) gives it as masculine [ὁ] and glosses it plainly "a dream," citing Iliad 2.80. But it records two further lives in the same breath. First, the dream is not only had but read: the phrase ὄνειρον ὑποκρίνεσθαι (oneiron hypokrinesthai), "to interpret a dream." Second, the dream is a person — LSJ notes ὄνειρος as a proper name, Dream personified, standing in Iliad 2.6 and following. And in the plural the Greeks gave the dreams a country: δῆμος ὀνείρων (dēmos oneirōn), "the people of dreams," at Odyssey 24.12.
No etymology pointer is matched in the record; the noun's deeper root is a gap this brief cannot honestly fill.
The cited surfaces lean hard on Aeschylus — six passages from the Agamemnon alone, where the word bends through its cases (ὀνείροις, ὀνειράτων, ὀνείρασιν, "in dreams," "of dreams") — with Xenophon's Anabasis and Epictetus's Discourses carrying it into prose. Everywhere it is a thing seen while unseeing, and then argued over.
If a dream in Greek is at once a message, a person, and a nation of its own, which of the three does the word most want to be?