The title of Luna is quite simple—it is the name that Regan's brother Liam has chosen to represent his true female self. It's an interesting title because it proclaims Liam's chosen name—instead of calling the book Liam, the author chooses Luna and, in doing so, honors the character's true identity. Additionally, luna means moon in Spanish, so the title is also a shout-out to the fact that for most of the book, Luna's true self only comes out at night.