What does 2-, 3- and 4-wire connection mean?

A resistance temperature detector (=RTD) can be connected with 2, 3 or 4 wires. The 2-wire connection needs only 2 wires, but then the measuring electronics is measuring also wire resistance, which is not desirable. Using extra wires for compensating the cable resistance gives more precisely the desired resistance of the RTD itself. The 4-wire connection is the most accurate one.