Line powered instrument is one that runs on line voltage e.g. a magnetic flow meter. Sometimes you hear the term 4 wire used although that can also cover an instrument that operates on 24 VDC
A loop powered instrument such as a pressure transmitter uses the power available from a 2 wire 4-20 mA loop to power it's electronics.
Note: Not all magnetic flowmeters are Line Powered, we are starting to see a few loop powered as well but generally they still operate from 120/230 AC or 24VDC.
Hope this helps
Roy
