Temperature Coefficient for a resistor characterizes how the observed resistance may change as a function of device temperature. Temperature coefficient can apply to other parts also like potentiometers, oscillators, crystals, RTD’s, and other components. If the component is a crystal instead of a resistor, then PPM will be referring to frequency instead of resistance.
Temperature coefficient is typically given in units of PPM (parts per million) per °C, which saves a lot of leading zeros as compared to using units of %/°C. Values may be positive or negative, indicating a direct or inverse correlation with temperature, respectively. It is a summary figure usually given in terms of a range or limiting maximum, and actual device behaviors will not necessarily vary linearly.
Notes from Riedon’s website
A resistor’s Temperature Coefficient of Resistance (TCR) tells how much its value changes as its temperature changes. It is usually expressed in ppm/°C (parts per million per degree Celsius) units. What does that really mean?
Let’s use an example: Riedon’s 100Ω 100/SM/PC Precision series resistor has a (standard) TCR of ±20ppm/°C. That means its resistance will not change more than 20/1,000,000 parts (or 0.0020%) per Ohm per degree Celsius temperature change (within the rated temperature range of -55 to +145°C, measured from 25°C room temperature.)
Assume our resistor is in a product that heats up from room temperature to 50°C. To find our resistor’s (maximum) ohmic change caused by that 25°C rise, multiply 0.0020% times 100 (the resistor value) times 25 (the temperature change.) The resistor’s value would change no more than 0.05Ω.
Because the temperature coefficient is the minimum and maximum possible deviations, the actual change may be much smaller, depending on the specific characteristics of that resistor. DigiKey can provide a TCR much lower than this example if you must guarantee a smaller resistance change in your application.
Additional Resources
Resistor Power Rating Derating and Temperature Coefficient (digikey.com)
Resistors in eewiki Motley Electronic Topics
DigiKey Resistor Selection