Aristotle's famous definition is precise: tragedy is an imitation of an action that is serious, complete, and of a certain magnitude, in language embellished with artistic ornament, presented in the form of action rather than narrative, and through pity and fear effecting the proper catharsis of those emotions. Every element of this definition has been debated for two millennia, but the final clause — catharsis through pity and fear — is the most discussed.
The Greek katharsis carries both medical and ritual meanings: the purging of bodily humours, and the cleansing of ritual pollution. Aristotle does not explain the term in the Poetics — he promises to do so in the Politics, though the passage is lost. Interpreters divide between those who read catharsis as emotional purgation (we discharge excess pity and fear in the safe space of drama) and those who read it as intellectual clarification (drama teaches us the proper objects of these emotions).
Catharsis must arise from the structure of the plot, not from stage effects. A tragedy that produces its emotional impact through spectacular means is exploiting the audience rather than moving them. The power of the Oedipus, Aristotle notes, is felt even in the reading — the plot alone is sufficient. This is why spectacle is the least artistic of tragedy's six elements, and why catharsis is a criterion for dramatic quality, not merely emotional intensity.
The definition of tragedy and catharsis appear in section VI (Chapter 2). Aristotle's promised fuller account in the Politics (Book VIII, Chapter 7) survives only in fragmentary form.