For chicken to be labelled organic it needs to meet several standards according to the USDA.
Certified organic chickens are fed organically grown feed. Additionally, USDA-certification for organic meat forbids use of growth hormones, antibiotics, genetically modified feed, or animal by-products in raising the animal. (Note than any chickens sold for meat in the U.S. can't be given hormones, so that doesn't differentiate organic chicken from conventionally raised chicken.)
What's so great about organic feed? Simple. It's made from grain that was raised without synthetic chemicals, irradiation, sewage sludge, or genetically modified organisms and the farm where it's grown is subject to inspections and detailed record keeping of practices used to farm the land.
What's more is the chickens need to have access to the outdoors. This access can be rather limited and can be denied in cases of severe weather, but it must exist. Organic chickens must also be raised on certified organic land, which means land that meets all the standards for growing organic crops, including being free from synthetic pesticide use for at least three years.
All animals destined to become certified organic meat must be raised in a way that promotes health and allows for natural behavior. They must have access to fresh water, room to exercise, fresh air, and sunlight.
All of these standards, including feed, must be in place by the second day of a chicken's life.
Many small farmers who raise chickens in traditional ways, with lots of room to roam and exhibit natural behaviors, may not be certified organic. Why not? Organically grown feed is much more expensive than conventionally grown feed. Many locavores prefer to buy chickens that may not have been fed organic feed, but have been raised on pasture - and thus allowed to hunt-and-peck on fields for much of their food.