A speedometer is the instrument used to measure the velocity of a moving vehicle. Velocity is a term we often hear when talking about speed, especially in vehicles. It tells us how fast something is moving and in what direction. In most cars, motorbikes, and even some bicycles, the device used to measure this is called a speedometer.

A speedometer measures how fast a vehicle is traveling at any given moment. It gives readings in kilometers per hour (km/h) or miles per hour (mph). The working principle behind it usually involves a magnetic or digital sensor that detects the rotation of the wheels or the drive shaft. This data is converted into a speed reading displayed on your dashboard.

Now let’s clear up why the other options are incorrect:
- **Anemometer** is used for measuring wind speed and is mostly used in weather stations.
- **Odometer** measures the total distance a vehicle has traveled over time, not its speed or velocity.
- **Hygrometer** measures humidity in the air, commonly used in weather forecasting and greenhouses.

Its also important to know that velocity is not just speed — it includes **direction**. However, for practical everyday uses, we often use speedometers to get a good sense of how fast were moving on the road, which serves as a basic velocity measurement tool in that context.

So, next time you are in a car, glance at the speedometer — its giving you real-time information about your velocity!