Dynamo is a word that used to simply mean an electric generator, but today a dynamo refers to a generator which produces direct current (DC) using a commutator, an electrical switch that generates electricity from the force of a rotating rod. Like any other generator, the purpose of a dynamo is to produce electricity from mechanical power. The source of the mechanical power could be a dam, windmill, or a coal or oil-burning power plant, with the latter being most common. Dynamos were quickly replaced by other electrical generators because of the advantages of alternating current (AC), inefficiencies of the commutator, and solid state methods for converting AC into DC power.

A dynamo is one of the simplest and earliest electrical generators, and the first used to generate power for industry. The famous inventor Thomas Edison believed that the future of electricity would be based on DC, driven by the dynamo, but it turned out that his eccentric rival, Nikola Tesla, was correct, and that electricity would be based on AC rather than DC, effectively dooming the dynamo to demonstrations in High School science classes.

The dynamo is fundamentally based on Faraday's law of induction, which states, "The induced electromagnetic force or EMF in any closed circuit is equal to the time rate of change of the magnetic flux linking the circuit." Basically, this means a current in a closed circuit can be induced when mechanical force is applied against the magnetic field linking the circuit, as in a generator, or vice versa, as in an engine. The first dynamo which was based on Faraday's law was built in 1832 by the French instrument maker, Hippolyte Pixii.

The dynamo led the first steps into the use of electricity in industry. Larger and larger dynamos were built, linked together in a series. The dynamo was not only the first commercially useful electrical generator, but also one of the first motors, which was discovered by accident. Today, the dynamo is mainly remembered as a simple device on which more complex, later electrical devices were based, such as the electric motor, the alternating-current alternator, and the rotary converter.