Field Of View abbreviated as FOV.
The FOV is that part of the night sky that is visible through the eyepiece, or on an image.
This is not to be confused with the "apparent FOV", which is the angle under which the FOV is seen in the eyepiece. The relation between the apparent FOV and the true FOV is as follows:
The FOV and Apparant FOV are specified in degrees or radians.
The maximal possible apparant FOV depends on the type of eyepiece. It ranges from about 45 degrees (Plössl) to about 80 degrees (Nagler).