Pretty simple. From the listening position what is the angle between your stereo speakers?
Might be nice to know the distance you sit from them as well. If you want to list speaker type that would be nice.
To prevent confusion some people might say plus and minus 30 degrees for speakers that create a total 60 degree angle. Or they might say the speakers form an angle of 60 degrees. Just be clear which way you are looking at it. Both of these illustrations are for the same speaker placement. 60 degrees or +/- 30 degrees.