You can define pressure drop any way you like, what ever is important for your application. But most people use the pressure drop from inlet to outlet, probably using the areaAve() function.

thanks a lot.

Glenn,, thanks a lot

Hi,
I measured the area avg pressure at inlet it gives me some value say 500 Pa.
and at outlet it gives O Pa. because of i have used outlet pressure Boundary condition to 1 atm (0 gauge).
then 500 is my pressure drop. is it write way?

Yes, that's right. You have a 500Pa pressure drop across your device.