Submental cervical angle
Where the underside of the chin meets the neck. The line people mean by 'jaw definition'.
The submental cervical angle is the angle between the underside of the chin and the front of the neck, measured in profile. It is the measurement behind most of what people mean by a defined jawline, because a jaw only reads as sharp when there is a clear line separating it from the neck. Surgical literature commonly cites 90 to 120 degrees as the aesthetically favourable range, with lower being sharper. Four things move it and only one is bone: submental fat, which changes with body composition; skin laxity, which changes with age; hyoid position, which is anatomy and does not change; and head posture, which changes the moment you lift your chin. That last one is why a before-and-after of this angle taken at two different head positions shows a transformation that did not happen.
This one cannot be measured from a front-facing photograph, so there is no tool for it here.