On CNC-machining centres, it usually refers to the router head. Often, the working spindle is equipped with a tool changing system to allow different processes or to hold angular aggregates.
Usually, the working range of a CNC machining center is described with the usable milling zones of the working spindle.
However, there is also the case that multiple router heads are available. Are these identical, they are usually called working spindles. If they are differently equipped spindles, there is usually one working spindle, which in contrast to another spindle may have a C-axis and a tool changer.