I consider the shoulder complex and the hip complex to be part of the core. It is irrelevant to me that NASM or any other group does not. If your shoulder is unstable and weak, it directly effects the function of the spine and can even effect stability at the feet and ankles.
We should be treating the body as a whole and not the sum of it’s parts.
All great responses. While the definition of “core” is obviously somewhat ambiguous I continue to suggest that it refers primarily to the muscles that support and stabilize the pelvis and lower spine. But I agree with Karin in that strengthening the shoulder girdle musculature has a profound effect on the function and strength of lower back and abdominal muscles. The integration of movement and strength throughout the entire spinal region is clearly an essential consideration in any training regimen. Don’t forget the shoulder internal and external rotators!
looking at all the answers that preceded my post, I personally include the training of the intrinsic stabilizers of the shoulder girdle in my definition of core training even though this is not according to textbook.