What is an on-line measurement? What is an off-line measurement?
On-line means that's it's generally continuous, or constantly taking the measure, through a tube connecting the batch. 

Offline is used for things that are taken out of the reactor, or removed from the process.  

Thus it's "on" or "off" the system depending on whether it's contained within the normal piping/fludics of the system: "On" is in the system, "Off" is out of the system.