Sinus Iridum?

I`ve been asked a few times - why Sinus Iridum? What does it mean?
I have a very amateur love for astronomy, mostly revolving around the collection of images (over 1500 of them to date and expanding rapidly thanks to Hubble and missions like Cassini), all categorised and tidy.

Sinus Iridum is the Bay of Rainbows on our moon.