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A large fluid-containing blister that exceeds 1 cm in diameter is classified as a bulla. Bullae are formed when fluid collects between the outer layer of skin and the second layer, resulting in a raised area filled with serous fluid. This characteristic distinguishes bullae from other skin lesions, as they are specifically defined by their size and fluid content. In contrast, fissures are small, linear cracks in the skin, while crusts refer to the hardened surface formed from dried exudate. Erosions are shallow losses of skin that do not penetrate through the entire thickness of the epidermis. Thus, the term bulla precisely describes the condition of a blister that is notably larger than 1 cm and is fluid-filled, demonstrating its unique properties in dermatological classifications.