Resistance
Symbol R, measured in ohms (Ω)
Resistance is how strongly something holds back the flow of current.
Resistance turns some electrical energy into heat. That is useful in a heater or toaster and wasteful in a long thin wire.
1 kΩ = 1,000 Ω and 1 MΩ = 1,000,000 Ω.
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Everything on this subject: Resistors, Simplified