Tragedy (event)
A tragedy is an event of great loss, usually of human life. Such an event is said to be tragic. Traditionally the event would require "some element of moral failure, some flaw in character, or some extraordinary combination of elements" to be tragic.
Not all death is considered a tragedy. Rather it is a precise set of symptoms surrounding the loss that define it as such. There are a variety of factors that define a death as tragic.
An event in which a massive number of deaths occur may be seen as a tragedy. This can be re-enforced by media attention or other public outcry.
A tragedy does not necessarily involve massive death. The death of a single person, e.g., a public figure or a child, may be seen as a tragedy. The person need not necessarily have been famous before death.
Factors that make death a tragedy
Generally, the label of "tragedy" is given to an event based on public perception. There are a number of factors that can make a death be considered a tragedy.
Scope
The scope of an event can affect the public view, and make it appear tragic. This can be the case whether the death toll is high, or if a single, unexpected death occurs in a well-beloved person.