Manual Mode
A mode, marked M, in which the photographer sets aperture and shutter speed directly and the camera only advises.
In depth
Manual is essential wherever the light is constant but the scene is not — studio flash, stage work, panoramas, time-lapse — because it guarantees frame-to-frame consistency that any automatic mode would destroy. Paired with Auto ISO it becomes a third semi-automatic mode, fixing both creative variables and floating only the gain.
Examples in practice
Shooting a stitched panorama in manual so that all twelve frames share identical exposure and blend without banding.