Any sideways twisting rotation along the lumbar vertebrae, and to a slight extent, thoracic vertebrae connected to floating ribs.

That is, the spine between the pelvis and ribcage.

As the axis of rotation is along the spine, a torso twist can be performed at the same time as a frontbend or backbend, or independent of either.

While twists of the neck might happen at the same time, they are not considered torso twists themselves.