Working Space
The colour space in which editing is performed, chosen for its gamut and neutrality rather than for display or output.
In depth
A good working space is device-independent and has neutral grey axes, so adjusting brightness does not shift hue. It should be at least as large as the widest output the file will ever face, since any colour clipped on entry cannot be recovered later. Editing in a wide space and converting on export is the standard discipline.
Examples in practice
Editing in ProPhoto at 16 bits and exporting to sRGB gives more headroom than editing in sRGB from the start.