Definition of "omicron"

Your Majestie knows that unfortunate I [...] was the first who when I published my Poem of Italia Liberata endeavoured to bring the Omicron, Eupselon, Omega, and other Greek Letters, which I thought very requisite for that language, into my own Tongue;

1656 , Henry, Earl of Monmouth, transl., I Raggvagli di Parnasso: or Advertisements from Parnassus, London: Humphrey Moseley, translation of De Ragguagli di Parnaso by Traiano Boccalini, page 180

The E serves both for the Epsilon and Eta; the O, both for the Omicron and Omega, and likewise for the dipthong OΥ.

1750 , James Russel, “Letter LI”, in Letters from a Young Painter Abroad to his Friends in England‎, volume II, London: W. Russel, page 120

... They include Mira itself (omicron Ceti) which can be seen shortly before dawn.

1992 July, Astronomy Now, No. 36, p. 1

There are now seven “variants of interest” or “variants of concern” and they each have a Greek letter, according to a W.H.O. tracking page. Some other variants with Greek letters do not reach those classification levels, and the W.H.O. also skipped two letters just before Omicron — “Nu” and “Xi” — leading to speculation about whether “Xi” was avoided in deference to the Chinese president, Xi Jinping.

2021 November 27, Vimal Patel, "How Omicron, the New Covid-19 Variant, Got Its Name", New York Times