What is the term dead ball mean in Cricket?
When will be a bowler's delivery considered as a dead ball?
Note that while a no ball and a wide ball are types of delivery, dead ball is a state. Put simply, the ball is not dead whenever it is in play.
According to Law 23.5:
The ball then will eventually become dead in different ways. The most common outcome is probably when the ball, after being played by the batsman, becomes "finally settled in the hands of the wicket-keeper or of the bowler" (Law 23.1). It is also dead, for instance, after a batsman is dismissed, a boundary is scored, or lost ball is called. There are further instances where umpires explicitly declare the ball to be dead: these include when a player is seriously injured, or when the bails fall from the striker's wicket before he plays the ball.
In relation to your second question, dead balls are counted as one of the over, unless:
This is merely a summary of the dead ball law (Law 23) and it is worth reading it to get a fuller understanding.