What activity or event(s) marks the point at which an economic "bubble" burst? What general activity or event(s) mark the time at which an economic "bubble" bursts? e.g. the dot-com bubble, etc.

Would it be considered to have "burst" when intrinsic value is less than price?
What about when this drop in intrinsic value might be due to some news event, etc... 

Is it simply when prices go down? When speculators exit the market, etc. or is it when "the something" happens that causes the drop in prices or the mass exodus from the market? Or something else altogether?
An economic "bubble" bursts when the buyers in the market deem the good or service to be overpriced.  

At a certain point, the price of whatever is causing the bubble will reach a certain "threshold price."  

It is at this "threshold price" that the corresponding amount of "threshold buyers" is reached.  These buyers will stop buying (or continue not to buy) the good or service offered.  

In an attempt to incentivize the buyers to continue buying their product, the sellers will lower their price.  Of course, certain sellers will lower their prices further to out maneuver each other in order to sell more of their product.  This is the burst.

At this point, some of the buyers will recognize the "burst" of the "bubble market" and will become sellers to move their product.  This, simultaneously, increases the amount of supply and decreases the demand, further lowering price.  

This cycle will continue until a new (lower) equilibrium is reached.