Effective aperture (Ae): It is the area over which the power is extracted from the INCIDENT wave and delivered to the load is CALLED effective aperture. 

Scattering aperture (As): It is the ratio of the reradiated power to the power DENSITY of the incident wave.

Loss aperture (Ae): It is the area of the antenna which dissipates power as heat. 

Collecting aperture (Ae): It is the addition of above three APERTURES.

PHYSICAL aperture (Ap): This aperture is a measure of the physical size of the antenna.

Effective aperture (Ae): It is the area over which the power is extracted from the incident wave and delivered to the load is called effective aperture. 

Scattering aperture (As): It is the ratio of the reradiated power to the power density of the incident wave.

Loss aperture (Ae): It is the area of the antenna which dissipates power as heat. 

Collecting aperture (Ae): It is the addition of above three apertures.

Physical aperture (Ap): This aperture is a measure of the physical size of the antenna.