The speed of the ball just after it is released from the hand. Speed is measured in MPH (miles per hour).
Higher velocity means the ball gets to the batter faster and requires quicker reaction by the batter. It will also help fielders get the ball to their target faster.
Real Life Example:
Changing pitch speeds, for example by mixing up fastballs and change-ups that have 5-10 mph difference in speed, can help to keep a batter off balance.