Sensors, signal conditioning equipment, data acquisition systems, controllers and control devices are all connected together to form a measurement/control loop or simply a loop in the industry.
Figure shows a typical measurement/control loop. The measurement loops make use of voltage, current or frequency signals, voltage or current being more common.
Measurement loops are configured in one of three ways: 2-wire, 3-wire and 4- wire.
Number of wires required for connecting power to the sensor and signal conditioning unit and carrying output signal to the rest of the equipment in the loop forms the basis of such classification.
https://engineerscommunity.com/t/what-are-two-wire-loops/9917/2
https://engineerscommunity.com/t/what-are-three-wire-loops/9916/2
https://engineerscommunity.com/t/what-are-four-wire-loops/9915/2