Plural of eta; the seventh letter of the Greek alphabet, represented as H or η.
From Greek 'eta' (ἦτα), which may derive from a Phoenician letter name. The plural simply adds the English '-s' to the Greek letter name.
Eta is one of those Greek letters that looks different depending on context—in math it's η, in English we write H—it's the same symbol that gives us the word 'ethos'!
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