I’m sure there’s an accepted medical definition in terms of attachments and flexion/extension of the peripheral skeleton but I would sum it as saying that core muscles work to align the pelvis, ribs and spine to resist either dynamic or static forces.
Core strength would be, in everyday terms, the ability of the body to maintain posture, resist force and initiate movement.
I’d class pelvic floor, diaphragm, transverse abs, obliques (internal and external), sacrospinalis & rectus abdominus (6 pack) as core muscles and also include lats, gluts and traps as having core function…
…hmmmm must get the anatomy books out again, I’m sure I’ve forgotten loads 😳