"RTD" stands for <b>R</b>esistance <b>T</b>emperature <b>D</b>etector. They're also called Platinum Resistance Thermometers (or whatever material they're made from). They're made of a material whose resistance changes in a predictable way and whose resistance at a given temperature is known.
long before sensors were placed in the windings and and in the bearing assemblies, the practice was to shut the motor off and measure the resistance of the various windings before the motor cooled down in order to gauge the temperature under load conditions.

later nickel rtd's were used. These days platinum rtds have found wider acceptance due to traceability of the measurement reading.