Film Grain
The visible texture produced by clumps of developed silver, which becomes more apparent with faster films and greater enlargement.
Measured in
RMS granularity
In depth
Grain is random and organic in a way digital noise is not, and it sits in the image rather than being overlaid on it, which is why the two look different even when matched for size. Faster films need larger crystals and so show coarser grain, and development choices — developer type, dilution, agitation, temperature — all modify its appearance.
Examples in practice
A 400-speed film enlarged to A3 shows a clearly visible texture that the same scene on 100-speed film does not.