15th Letter of the Greek Alphabet
Puzzle 12 of Dungeon Delving Pack
Question is: 15th Letter of the Greek Alphabet
and the answer is: OMICRON
Word Meaning:
OMICRON — (sometimes capitalized) The 15th letter of the Classical and Modern Greek alphabets (16th in Ancient and Old Greek), used in ordering lists as in naming (astronomy) the 15th star of a constellation or (epidemiology) the 15th discovered major variant of a disease.
Definitions from Wiktionary.