Sorry but this is not a secret way of defining the core, it’s a matter of me defining what I feel the core is.
For the last few decades, the term “core” has generally been associated with the abdominals and lumbar (lower back) region. Some folks may have even included the hips and thighs as part of the trendy “core” area as well but that’s neither here nor there. The trend in training the core has created an overly useless obsession with “guts and butts” classes, countless exercises addressing the abdominals, workout routines lasting not minutes but upwards of an hour all in the name of having a defined midsection. Hell, more and more people are suffering from weakened abdominals and having lower back issues due to the absurdly overused core training. Yes, the midsection/core is an important region to address, but guess what, so is the rest of the body. To me, the “core” is defined as the whole body: the front, back, left and right sides, top to bottom. All, yes all of your muscles and joints need attention and not specialized classes that make you “feel the burn”.
I find it a bit ridiculous to spend an inordinate amount of effort on an area that needs little attention – especially when a solid structured strength program is in place. The abdominals in particular work to stabilize the spine and if you believe that static work can strengthen a muscle (and it does), the abs and lower back are getting quite a stimulation when you are engaging other body parts. Hell, do a set of negative chins and see how sore your abs (along with chest, shoulders and triceps) are the next day from stabilizing you throughout the movement.
Am I suggesting that you not train your abs directly? No (although if you didn’t it would be the end of the world because no amount of ab work will whittle-the-middle unless you reduce body fat), but I am suggesting that you give more of your attention and energy towards training your entire body and treat your abs and lower back like any other muscle group.