This is a posh word for an introduction, preface or foreword or, to counter ponderous Greek with obscure Latin, an exordium.
It’s the neuter of the present participle passive of the Greek prolegein, to say beforehand, and is much rarer than its relative prologue, which is from Greek prôlogos, literally “fore-speech”. Both have travelled via Latin to reach us, but prologue has shuffled off its high-flown classical links while prolegomenon is condemned by its length and shape to be reserved for grand-sounding intellectual occasions and formal scholarship.
As prolegomenon to the systematic account of what I regard as the truth about the history of psychiatry presented in this book, I offer Roy Porter’s restatement of the premises that underlie my writings on this subject.
Coercion as Cure, by Thomas Szasz, 2007.
It may be a prologue to a book but it may also describe a work that introduces the study of a subject. A perusal of the dustier shelves of a large library may find titles such as A Prolegomenon to the Material Culture of Garments in the Formative Islamic Period, A Prolegomenon to the Reconceptualisation of Dialectic and Prolegomenon for an Excuse-Centered Approach to Transitional Justice.
Should you ever need to discuss more than one prolegomenon, the plural is prolegomena, though this has also at times been used irregularly for the singular, especially in what is almost certainly the best known use of the word, in Immanuel Kant's Prolegomena to Any Future Metaphysics That Will Be Able to Present Itself as a Science of 1783, a summary and introduction to his Critique of Pure Reason of two years previously.