resistor
/ rih-ZIS-ter /
A resistor is a small circuit component made on purpose to have a definite, reliable resistance. If resistance is the property, a resistor is the part you buy that carries a chosen amount of it, the way a weight in a set is manufactured to be exactly one kilogram. It answers a builder's need: I want to hold the current to a safe value or split a voltage, so give me a part with precisely 470 ohms.
Precisely, a resistor is a two-terminal device engineered to obey V = I R with a stated value of R, usually made of a carbon film, metal film, or a coil of resistance wire. Its value is marked by coloured bands or printed numbers, along with a tolerance (how close the real value is to the label) and a power rating (how much heat it can shed before it is damaged, in watts). Most ordinary resistors are ohmic, so the current through them is simply I = V / R.
You use resistors constantly: to limit current so a delicate LED is not fried, to divide a voltage down to a level a sensor can read, to set the timing of an RC circuit, and to deliberately turn electrical energy into heat. One honest caution: a resistor does not 'use up' current, so the same current that enters one leaves it; what it removes is energy, dissipated as heat at a rate P = I^2 R. Push too much current and it overheats past its power rating and burns out.
To run a red LED (which needs about 2 V and 20 mA) from a 5 V supply, you place a resistor in series to drop the extra 3 V. The needed value is R = V / I = 3 / 0.02 = 150 ohms.
A resistor is a packaged, labelled chunk of resistance you drop into a circuit to control current.
A resistor removes energy (as heat), not current. The same current that flows in flows out; every resistor also has a power rating you must not exceed or it burns out.