Quick answer
It is the tightest curve the cable will take without hidden damage, and it is quoted as a multiple of the cable's overall diameter, written as a number times D. Bend tighter than that and the insulation thins on the outside of the bend while the screen wrinkles inside, and the cable can fail there months later even though it tested fine.
As a rough guide, in line with IEC 60502-1, LV multicore cables sit around 12 × D, single-core LV around 15 × D, and screened MV and HV cables from 15 up to 20 × D. The diameter is the whole cable, armour and all, not the conductor. During pulling keep it gentler still and use rollers. The number printed on the drum always overrides any rule of thumb.