"出口截面面积" can be translated into English as "exit cross-sectional area".
Here, "exit" refers to the outlet or the point where something exits, and "cross-sectional area" is the area of a cross-section, which is a plane figure obtained by cutting a solid with a plane. In the context of fluid dynamics, engineering, or physics, this term is commonly used to describe the area of the opening through which a fluid, gas, or other substance exits a system or a container.