Velocity
Definition
Speed with a direction attached. 60 km/h north is a velocity; 60 km/h by itself is only a speed.
Symbol: · Unit: meters per second (m/s)
Technical definition
The time rate of change of displacement, v = Δx/Δt, a vector quantity. Instantaneous velocity is dx/dt, the derivative of position with respect to time.
Example
A cyclist riding 12 m/s east has a velocity of +12 m/s if east is taken as positive. Turning around at the same pace makes the velocity −12 m/s, even though the speedometer still reads 12 m/s.
Velocity is not speed. A race car lapping a circular track at a steady 200 km/h has constant speed but constantly changing velocity, because its direction keeps changing — and changing velocity means it is accelerating, even with cruise control locked. Whenever direction matters to the answer, you need velocity, not speed.