A citrus-specific term for the exocarp (the outermost layer of a fruit). This is where the oils and pigments are.
Basically, it's another word for zest, or what zest is before it's scraped off the citrus.
Etymology: I haven't seen a specific word origin, but other similar words (flavin, flavone, flavonoid) all originated from the Latin flavus, yellow.
(Exocarp is from the Greek exo (outside) + karpos (fruit).)
In an orange, the flavedo contains both orange pigment (carotenoid) and green pigment (chlorophyll) in its cells. The green pigment dominates. But the flavedo also contains an enzyme that will destroy the chlorophyll, separated from it by a thin membrane.
In chilly weather, the membrane breaks down, the enzyme destroys the chlorophyll, and the orange turns orange-colored. But that only happens if the orange is on the tree; a green orange removed from a tree will stay green, unless it is "gassed" (exposed to ethylene gas, the same gas that's given off by a ripening apple or banana) or bathed in dye.
Around here, we're used to oranges being a very intense orange color. But in other areas, especially places where it doesn't get that cool, green oranges might be more common. The color has nothing to do with how ripe the orange is. Yet the name for the color was derived from the name for the fruit, which I always find confusing.
John McPhee's Oranges says growers originally thought kerosene stoves helped to turn the color on oranges (they did, but just because of the ethylene gas they gave off). Oranges were placed in "sweat rooms" with banks of stoves, which left the oranges smoked and dehydrated, and sometimes caused whole packinghouses to burst into flames.