Q:

When will a bowler's delivery be declared as a dead ball?

What is the term dead ball mean in Cricket?
and
When will be a bowler's delivery considered as a dead ball?

A:

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 ceases to be dead - that is, it comes into play - when the bowler starts his run up or, if he has no run up, his bowling action.

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:

the striker has not had an opportunity to play it (for instance, if the bowler drops the ball during his run up)
the striker was not ready (with a good reason) and did not attempt to play the ball (for instance, if the batsman pulls away due to movement behind the bowler's arm, and the bowler still bowls the ball)

This is merely a summary of the dead ball law (Law 23) and it is worth reading it to get a fuller understanding.

