Electrical resistance measures a material's opposition to the flow of electric current. The SI derived unit is the ohm (Ω), defined as the resistance between two points when a potential difference of one volt causes a current of one ampere: Ω = V/A = kg·m²·s⁻³·A⁻². Named after Georg Simon Ohm, who formulated Ohm's Law (V = I × R) in 1827.
Resistor values in electronics range from milliohms (wire shunts for current sensing) through kilohms (signal circuits) to megaohms (high-impedance inputs, ESD protection). The colour-band coding on resistors encodes values in ohms; standard series (E12, E24, E96) cover common values. Resistance also changes with temperature — a key factor in thermistors and precision measurement circuits.
| 1 kΩ | = 1000 Ω |
| 1 MΩ | = 1,000,000 Ω |
| 1 Ω | = 1000 mΩ |
| Typical LED resistor | 220–470 Ω |
| Pull-up resistor | 10 kΩ |
| Human body (dry) | ~100 kΩ |