The name Senna is a girl's name of Arabic origin meaning "brightness". Senna and is often added to lists like Flower Names for Girls and discussed in our forums with posts like "Girls Makeover - Two Middles".
From the experts:
Senna is a flowering bush common in the tropics, with bright yellow flowers. In herbal medicine, Senna has gained some notice as the active ingredient in so-called "dieter's teas," which works mainly as a laxative. While that is a less-than-savory association, Senna has a pretty sound and is a fresh entry in the flourishing botanical names genus.