What is the meaning of Omicron?

The 15th letter of the Classical and Modern Greek alphabets (16th in Ancient and Old Greek), used in ordering lists as in naming the 15th star of a constellation or the 15th discovered major variant of a disease.

Ellipsis of Omicron variant.

omicron (Greek letter)

omicron (Greek letter)

omicron (Greek letter)

Source: wiktionary.org