Visual angle is defined as angle between the straight lines of eye's lenses. It will define the size and lenght of stimulus on screen.
This measurement is used to correct (standardize) the size of stimulus which is more relyable to measure it this way than actuall size for example in cm. If you present 100 px stimulus on screen it is 3°, but if you sit far from monitor it could be for example 2°. This is important that visual angle measurment reflects a disatnce from screen.
Michael Tesar, 2017
Based on Sebastiaan Mathôt OpenSesame documentation here.