What does surface area refer to in geometric solids?

Surface area refers to the combined area of the surface of a geometric solid. This means it encompasses all the external surfaces of the solid, providing a measure of the total area that makes up the outside of the shape. For example, when calculating the surface area of a cube, you would find the area of each of the six faces and sum them up to get the total. This concept is vital in various applications, such as determining the amount of material needed to cover an object or the heat exchange between the object and its environment.

Other options do not accurately represent what surface area is. The total volume within the solid pertains to the space inside it, while the area of only the bases would not include the sides and top of the solid. The area of the solid's outline suggests a line representation rather than a true measure of the surface area, which requires consideration of flat surfaces. Therefore, the definition given by the correct choice thoroughly captures the essence of what surface area entails.