Here's the official Disney definition:
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•Dress Code Guidelines:
Men: Khakis, slacks, jeans, dress shorts, collared shirts. Sport coats are optional.
Ladies: Capris, skirts, dresses, jeans, dress shorts.
Not permitted in dining room: Tank tops, swimwear, hats for gentleman, cut offs, or torn clothing. While T-shirts are now allowed, the policy remains that T-shirts with offensive language or graphics are not acceptable.
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And Boma -indeed, all non-signature restaurants in or out of the parks - are very much "come as you are," with your park clothes on. Obviously, you don't want to come in shirtless or with a swimsuit on. The only exception I'm aware of is Bistro de Paris in Epcot, which wants you to be a little dressier. And the guidelines for some of the "non-Disney" options, such as Shula's and BlueZoo at the Swan/Dolphin, might vary some still.