A speedometer is a device used to indicate the instantaneous speed of a vehicle.
The speedometer in the figure above indicates speed in both miles per hour (black) and kilometers per hour (red). The speedometer also has an electronic odometer which displays the number of miles traveled in the digital display (58.2 miles).
Most modern speedometers, like the ones you see in a car, are electronic. They typically also include an odometer, which measures the distance traveled. Through use of the odometer, it is possible to measure average speed as long as you also keep track of time.
Electronic speedometers involve rotation sensors that deliver electronic pulses corresponding to the average rotational speed of a car's driveshaft, and therefore the vehicle's speed. The frequency of the pulses is converted into a speed by a computer, which is then displayed on the interface of the speedometer, typically using an analog-style needle. The speedometer can also have a digital display. In some cases, both analog and digital displays are used.
Bicycles can also have speedometers. These work slightly differently than the electronic speedometers commonly used in cars. Most typically, bicycle speedometers measure the time between the revolution of each wheel through use of a sensor that is fixed to some point on the bicycle, as well as a magnet attached to the spoke. As the wheel rotates and the magnet passes the fixed sensor, the sensor pulses, allowing for the measurement of speed based on the time between each revolution. The speed is then usually indicated on a digital display mounted on the handlebar of the bike.