The term 'windmill blade' is composed of two parts: a VANE (4 letters) and a SAIL (4 letters). A VANE is a flat surface that is attached to the hub of a windmill and is used to catch the wind. The SAIL is a curved surface that is connected to the VANE and is used to capture the wind's energy and turn the blades of the windmill. Together, the VANE and SAIL form the windmill blade, which is used to generate power from the wind.