Temperature co-efficient refers the change of resistance with respect to temperature of a substance. If it's resistance increased with temperature, then the characteristics of the substance is called as positive temperature coefficient. For example: any metallic conductor (Cu, Fe). On the other hand if it's resistance decreased with temperature, then the characteristics of the substance is called as negative temperature coefficient. For example: insulators and semiconductors. Otherwise, If it's resistance doesn't vary with temperature, then the characteristics of substance is called as zero temperature coefficient. Actually there is no absolute zero temperature coefficient substance. But resistance of materials like eureka, manganin, constantan etc. have changed negligible (almost zero) with temperature.