Turndown and rangeability is usually associated with control valves. The definition of these are (from Perry):
Rangeability, R= maximum controllable flow / minimum controllable flow
Turndown ratio, T= normal maximum flow / minimum controllable flow
Generally valves are designed so that T=0.7*R where R is between 20 and 50.
This definition complies with the ISA standard.
Turndown is often used not only in connection with control valves but also as a more “global” parameter e.g. for whole plants.