Iris, Iridis

Iris, Iridis (noun)
– declension: 3rd declension
– gender: feminine
1. Iris (messenger of the gods, goddess of the rainbow)
2. rainbow

You can identify third declension nouns by their genitive singular ending ‘-is’. See and Wikipedia.

In contrast with the first- and second-declension endings, those of the third declension lack a theme vowel (a or o/u in the first and second declensions) and so are called athematic.

Note via Toby O.