Neumann and Sengpiel are broadly in agreement that it’s a measure of orchestral image width between the speakers (throw away your headphones !)

Williams is correct in his precision of definition… but Sengpiel’s interactive app lets you toy with the notion of images bunching up in the centre, being evenly spread across the L/R spectrum, or clustered around each speaker leaving a hole in the middle.

Try the 3 approaches and commit to the one that gives you the best results…

I too have suspicions about the rigour of Sengpiel’s testing methodology…so why not put that free Neumann app on your cellphone immediately, then start playing around with the variables !