The first great theorist of dramatic art was Aristotle (384-322 B.C.) whose discussion of tragedy in Poetics has set the bar for what constitutes tragedy.

• According to Aristotle, tragedy is the imitation in dramatic form of an action that is serious and complete, with incidents arousing pity and fear wherewith it effects a catharsis of such emotions (an emotional release) at the end. 
• The best tragic plots involve a reversal of fortune or a discovery (change from ignorance to knowledge) or both.

Here is a description of the tragic protagonist:

1. The tragic hero is a man/woman of noble stature.
2. The tragic hero is good, though not perfect, and his/her fall results from his/her committing what Aristotle calls "an act of injustice" (hamartia) either through ignorance or from a conviction that some greater good will be served.  The protagonist is personally responsible for his/her downfall.
3. The hero's downfall is tragic rather than merely pathetic. 
4. Nevertheless, the hero's misfortune is not wholly deserved.  He/She is a person mainly admirable, and the fall, therefore, fills the audience with pity and fear (catharsis).
5. The tragic fall is not pure loss.  He/She exits accepting his/her fate and acknowledging that it is to some degree just.
6. Tragedy does not leave its audience in a state of depression.

It is important to note that there is a critical tradition that attributes the fall of the hero to a "tragic flaw"--some fault of character such as inordinate ambition, quickness to anger, a tendency to jealousy, or overweening pride.  This definition seems more appropriate to "Macbeth" as you have tagged your question as his inordinate ambition (along with Lady Macbeth's influence) is the driving force for his downfall.