Velocity is a fundamental concept in kinematics, which is the branch of classical mechanics that describes the motion of bodies. It is defined as the speed and direction of motion of an object. Velocity is a physical vector quantity, which means that both magnitude and direction are needed to define it. The scalar absolute value (magnitude) of velocity is called speed, which is a coherent derived unit measured in the SI (metric system) as meters per second (m/s or m⋅s−1) . For example, "5 meters per second" is a scalar, whereas "5 meters per second east" is a vector. If there is a change in speed, direction, or both, then the object is said to be undergoing an acceleration.

The difference between speed and velocity is that speed is a scalar quantity that denotes only how fast an object is moving, while velocity is a vector quantity that tells us both the speed and direction of the objects motion. Instantaneous velocity is the velocity of a body at any given time. Velocity is direction-aware, so when evaluating the velocity of an object, one must keep track of direction. It would not be enough to say that an object has a velocity of 55 mi/hr; one must include direction information to fully describe the velocity of the object.

The unit of measurement for velocity, just like speed, is "meters per second". The SI unit of measurement for velocity is meters per second (m/s), but many other units such as miles per hour (mph) and kilometers per hour (km/h) are commonly used. The established SI unit of measurement for velocity is meters per second because it helps everyone in the world adhere to one common style of measurement.

In conclusion, velocity is a vector quantity that describes the speed and direction of motion of an object. It is direction-aware and is measured in meters per second (m/s).