Which term describes lesions that are flat and less than 0.5 cm in diameter?

The term that describes lesions that are flat and less than 0.5 cm in diameter is macules. Macules are defined as flat, distinct, and non-palpable lesions that do not protrude above the skin's surface. They can vary in color compared to the surrounding skin, but they are characterized by their lack of elevation and are typically smaller than 0.5 cm.

Vesicles, on the other hand, are small fluid-filled blisters that are elevated and larger than macules. Patch lesions refer to larger flat lesions that exceed 0.5 cm and may have a similar appearance to macules but are larger. Papules are small, raised bumps on the skin that are also typically less than 0.5 cm in diameter, but they are elevated above the surface, distinguishing them from macules.

Thus, the correct term for flat lesions less than 0.5 cm in diameter is indeed macules.