PPM per degree Centigrade

A resistor's Temperature Coefficient of Resistance (TCR) tells how much its value changes as its temperature changes. It is usually expressed in ppm/℃ (parts per million per degree Centigrade) units. What does that really mean?
Let's use an example: Riedon's 50 Ω resistor has a (standard) TCR of 20ppm/ohm/℃. That means its resistance will not change more than 0.000020 ohms (20.1,000,000) per ohm per degree Centigrade temperature change (within the rated temperature range of -55 to +145℃, measured from 25℃ room temperature.)
Assume our resistor is in a product that heats up from room temperature to 50℃. To find our 50W resistor's (maximum) change caused by that 25℃ rise, multiply 20ppm times 50Ω times 25 (the temperature change.)
$$0.000020 \times 50 \times 25 = 0.025W.$$
For resistors, a 5000 PPM level is considered high, whereas 20 PPM is quite low. A 1000 PPM/℃ characteristic reveals that a $1^{\circ}$ change in temperature results in a change in resistance equal to 1000 PPM, or $${1000\over 1,000,000}= {1 \over 1000}$$ of its nameplate value-not a significant change for most applications. However, a 10℃ change results in a change equal to 1/100 (1%) of its nameplate value, which is becoming significant.
In equation form, the change in resistance is given by $$\Delta R = {R_{nominal} \over 10^6} (PPM)(\Delta T) $$ where $R_{nominal}$ is the nameplate value of the resistor at room temperature and $\Delta T$ is the change in temperature from the reference level of 20℃.