What is the unit of electrical resistance?

The ohm, named for Georg Ohm, whose 1827 work established that current is proportional to voltage - and which was dismissed on publication. He was a schoolteacher without a university position, the German physics establishment regarded the approach as unphilosophical, and he resigned his post in disappointment. Recognition arrived about twenty years later.

The unit's symbol is a Greek capital omega, which is a small piece of engineering pragmatism - the letter O would be unreadable next to zeros in a circuit diagram. The reciprocal, conductance, was for a long time measured in mhos, ohm spelled backwards, which is the kind of joke engineers make and which was eventually replaced by the more respectable siemens.

Resistance is also why anything electrical gets warm. Energy lost to it becomes heat, which is a nuisance in a phone and the entire point of a kettle, a toaster and an incandescent bulb. And it is why long-distance power lines run at extremely high voltage: for the same power delivered, higher voltage means lower current, and losses go as the square of current - so raising the voltage is the difference between a viable grid and cooking the countryside.