Ball speed?

it means, the force at which the bat hits the ball is known as the bat speed. i think it's a term for the modern day cricket like the 20-20 because there are many sixes and fours in these type of matches. it is the speed of the bat at which it comes in contact with the ball.

Its the same as it sounds. The speed of the ball after hitting the bat. It is usually compared against fast bowlers. The commentators say it if they feel the ball was traveling faster after hitting the bat when compared with the speed at which the ball was bowled. If the batsman defends the ball then the ball speed off the bat will be low. If the batsman smashes the ball then it is likely to be faster.