far-field region
The region where the angular field distribution is essentially independent of distance from the source.
Note 1: If the source has a maximum overall dimension D that is large compared to the wavelength, the far-field region is commonly taken to exist at distances greater than 2D2/ 
from the source, being the wavelength.
Note 2: For a beam focused at infinity, the far-field region is sometimes referred to as the Fraunhofer region. Synonyms far field, far zone, Fraunhofer region, radiation field.