Definition: the final action that completes the unraveling of the plot in a play, especially a tragedy
The most commonly used sense of catastrophe today is “a terrible disaster.” However, when the word first entered our language in the 16th century, it was as a theatrical term meaning "the conclusion or final event of a dramatic work." Since the catastrophe was most often used in reference to a tragedy, it quickly extended its meaning to be applied to any sort of unhappy ending, and within a few more centuries had come to take on its present sense.
Thus hauing boldly importuned your assistance, and tediously molested your eares with circumstances, leauing now at length to abuse your friendly pacience … and after the Catastrophe of this worldly Comedie, (wherein you play a statelye parte) the gladsome ioyes of the euerlasting Seignorie.
—George Turberuile, Introduction to translation of Ovid’s The Heroycall Epistles of the Learned Poet Publius Ouidius Naso, 1567