The meaning of the term is highly controversial, but in relation to the drama it generally refers to an emotional result that derives from strong feelings such as great sorrow, fear, pity, laughter, or any other extreme change in emotion; this result has traditionally been variously described as either a purification or a purging of such emotions (whether those of the characters in the play or of the audience is also controversial).  More recently such terms as restoration, renewal, and revitalization have been used in relation to the effect on members of the audience.
Using the term "catharsis" to refer to the emotions was first done by the Greek philosopher Aristotle in his work Poetics. It refers to the sensation, or literary effect, that would ideally overcome either the characters in a play, or an audience upon finishing watching a tragedy (a release of pent-up emotion or energy). In his previous works, he used the term in its medical sense (usually referring to the evacuation of the "katamenia", the menstrual fluid or other reproductive material). Because of this, F. L. Lucas maintains that catharsis cannot be properly translated as purification or cleansing, but only as purgation. Since before Poetics catharsis was purely a medical term, Aristotle is employing it as a medical metaphor. "It is the human soul that is purged of its excessive passions." Lessing sidesteps the medical aspect of the issue and translates catharsis as a purification, an experience that brings pity and fear into their proper balance: "In real life," he explained, "men are sometimes too much addicted to pity or fear, sometimes too little; tragedy brings them back to a virtuous and happy mean." Tragedy is then a corrective; through watching tragedy the audience learns how to feel these emotions at the proper levels. Some modern interpreters of the work infer that catharsis is pleasurable because audience members felt ekstasis (Greek: ἔκστασις) (ecstacy) (literally: astonishment, meaning: trance) from the fact that there existed those who could suffer a worse fate than them was to them a relief. Any translator attempting to interpret Aristotle's meaning of the term should take into account that Poetics is largely a response to Plato's claim that poetry encourages men to be hysterical and uncontrolled. In response to Plato, Aristotle maintains that poetry makes them less, not more, emotional, by giving a periodic and healthy outlet to their feelings.
In literary aesthetics, catharsis is developed by the conjunction of stereotyped characters and unique or surprising actions. Throughout a play we do not expect the nature of a character to change significantly, rather pre-existing elements are revealed in a relatively straight-forward way as the character is confronted with unique actions in time. This can be clearly seen in Oedipus Rex where King Oedipus is confronted with ever more outrageous actions until emptying generated by the death of his mother-wife and his act of self-blinding.
In contemporary aesthetics catharsis may also refer to any emptying of emotion experienced by an audience in relation to drama. This exstasis can be perceived in comedy, melodrama and most other dramatic forms. Deliberate attempts, on political or aesthetic bases, to subvert the structure of catharsis in theatre have occurred. For example, Bertolt Brecht viewed catharsis as a pap for the bourgeois theatre audience, and designed dramas which left significant emotions unresolved, as a way to force social action upon the audience. In Brecht's theory, the absence of a cathartic resolving action would require the audience to take political action in the real world in order to fill the emotional gap they experience. This technique can be seen as early as his agit-prop play The Measures Taken.
First of all, the tragic catharsis might be a purgation. Fear can obviously be an insidious thing that undermines life and poisons it with anxiety. It would be good to flush this feeling from our systems, bring it into the open, and clear the air. This may explain the appeal of horror movies, that they redirect our fears toward something external, grotesque, and finally ridiculous, in order to puncture them. On the other hand, fear might have a secret allure, so that what we need to purge is the desire for the thrill that comes with fear. The horror movie also provides a safe way to indulge and satisfy the longing to feel afraid, and go home afterward satisfied; the desire is purged, temporarily, by being fed. Our souls are so many-headed that opposite satisfactions may be felt at the same time, but I think these two really are opposite. In the first sense of purgation, the horror movie is a kind of medicine that does its work and leaves the soul healthier, while in the second sense it is a potentially addictive drug. Either explanation may account for the popularity of these movies among teenagers, since fear is so much a fact of that time of life. For those of us who are older, the tear-jerker may have more appeal, offering a way to purge the regrets of our lives in a sentimental outpouring of pity. As with fear, this purgation too may be either medicinal or drug-like.
This idea of purgation, in its various forms, is what we usually mean when we call something cathartic. People speak of watching football, or boxing, as a catharsis of violent urges, or call a shouting match with a friend a useful catharsis of buried resentment. This is a practical purpose that drama may also serve, but it has no particular connection with beauty or truth; to be good in this purgative way, a drama has no need to be good in any other way. No one would be tempted to confuse the feeling at the end of a horror movie with what Aristotle calls “the tragic pleasure,” nor to call such a movie a tragedy. But the English word catharsis does not contain everything that is in the Greek word. Let us look at other things it might mean.
Catharsis in Greek can mean purification. While purging something means getting rid of it, purifying something means getting rid of the worse or baser parts of it. It is possible that tragedy purifies the feelings themselves of fear and pity. These arise in us in crude ways, attached to all sorts of objects. Perhaps the poet educates our sensibilities, our powers to feel and be moved, by refining them and attaching them to less easily discernible objects. There is a line in The Wasteland, “I will show you fear in a handful of dust.” Alfred Hitchcock once made us all feel a little shudder when we took showers. The poetic imagination is limited only by its skill, and can turn any object into a focus for any feeling. Some people turn to poetry to find delicious and exquisite new ways to feel old feelings, and consider themselves to enter in that way into a purified state. It has been argued that this sort of thing is what tragedy and the tragic pleasure are all about, but it doesn’t match up with my experience. Sophocles does make me fear and pity human knowledge when I watch the Oedipus Tyrannus, but this is not a refinement of those feelings but a discovery that they belong to a surprising object. Sophocles is not training my feelings, but using them to show me something worthy of wonder.
The word catharsis drops out of the Poetics because the word wonder, to rhaumaston, replaces it, first in chapter 9, where Aristotle argues that pity and fear arise most of all where wonder does, and finally in chapters 24 and 25, where he singles out wonder as the aim of the poetic art itself, into which the aim of tragedy in particular merges. Ask yourself how you feel at the end of a tragedy. You have witnessed horrible things and felt painful feelings, but the mark of tragedy is that it brings you out the other side. Aristotle’s use of the word catharsis is not a technical reference to purgation or purification but a beautiful metaphor for the peculiar tragic pleasure, the feeling of being washed or cleansed.