Definition of "exordium"

Cicero thinks, in discourses of philosophy, the exordium to be the hardest part: if it be so, I wisely lay hold on the conclusion.

1603, Michel de Montaigne, chapter 17, in John Florio, transl., The Essayes […], book II, London: […] Val[entine] Simmes for Edward Blount […]

The depreciation of her produce was next insisted upon; and I found this exordium led to the information that Messrs. Standish and Co. had been enabled, from the depressed state of the market, to lay in a large stock of Irish linen at unheard-of low prices.

1831, L[etitia] E[lizabeth] L[andon], Romance and Reality. […], volume I, London: Henry Colburn and Richard Bentley, […], pages 180–181

This is a feeble article of faith to begin with, but it helps to push my pen through this exordium and what now follows.

1985, Anthony Burgess, Kingdom of the Wicked