A dynamo is a machine that converts motion into electricity. Picture a bicycle with a small generator attached to its wheel: as you pedal and the wheel spins, the dynamo produces electricity to power the headlight. Before modern batteries became common, dynamos were an important way to generate electrical power for things like lightbulbs and factory machinery.

Dynamos literally create electricity from movement. In many power plants, large generators based on the same principles spin turbines to generate electricity for entire cities. Smaller dynamos might power the lights on a bike or charge a hand-cranked radio.

People also use dynamo to describe someone with exceptional energy and drive. If your friend finishes her homework, practices violin, helps with dinner, and still has energy for a game of tag, someone might call her a dynamo. This meaning captures the idea of someone who seems to generate their own power and keeps going when others run out of steam. Like the machine it's named after, a human dynamo turns effort and enthusiasm into impressive results.