平坦 (píng tǎn) specifically describes a surface that is flat and level, like a road or ground, with no bumps or slopes. It differs from 平 (píng), which is broader (also 'calm, peaceful'), and from 扁 (biǎn), which means 'flat' in the sense of thin and wide (like a coin). 平稳 (píng wěn) focuses on smoothness and stability during movement, not the surface itself.