How is the declension of ἠχώ categorised?

Greek Learning Greek
1

Which declension does ἠχώ, ἠχοῦς, ἡ belong to?

2

3rd (consonant) declension. Stem originally in -οι. See Smyth 279:

  1. Stems in οι, with nominative in -ώ, turn ι into unwritten ι-glide (y) before the endings beginning with a vowel. ἡ πειθώ persuasion is thus declined:

N. πειθώ. G. πειθοῦς (πειθό-ος). D. πειθοῖ (πειθό-ι). A. πειθώ (πειθό-α). V. πειθοῖ. Dual and plural are wanting.

So ἠχώ echo, εὐεστώ well-being, φειδώ sparing, Σαπφώ, Λητώ, Καλυψώ. οι stems are chiefly used for women’s names.

a. A stronger form of the stem is ωι, seen in the earlier form of the nominative (Σαπφῴ, Λητῴ). The accusative has the accent of the nominative.

b. When dual and plural occur, they are of the second declension: nom. λεχοί (late) from λεχώ woman in child-bed, acc. γοργούς from γοργώ gorgon.

c. ἡ εἰκών image, ἡ ἀηδών nightingale, properly from stems in ον, have certain forms from this declension (εἰκοῦς, εἰκώ, voc. ἀηδοῖ).

http://www.perseus.tufts.edu/hopper/text?doc=Smyth+grammar+279&fromdoc=Perseus%3Atext%3A1999.04.0007

3

Correct me if I’m wrong, the omega is simply the same lengthening for the nominative as for χιών (gen. χιόνος) and many other words.