IMO, Decentralized Decision Making is one of the fundamental characteristics of a flat organizational structure. Nowadays, at many places, 'flat structure' is being wrongly understood as 'easily approachable management'. However, such companies don't understand and don't conduct decentralized form of decision making. I would not prefer to go after literal meaning of the word 'flat', but at my organization, if I am able to conduct decentralized form of decision making in an effective manner, I would be happy to call it a 'flat organization'.