introduction (to a lengthy or scholarly writing or speech)  n.: prolegomenon

in All this, however, is by way of a prolegomenon to the book’s real work, which is to show what feminist thought has to contribute to bioethics. (Hilde Lindemann Nelson, Feminist Approaches to Bioethics: Theoretical Reflections and Practical Applications [Review], Hypatia, 09/22/1998, p. 112.)

[John Kennedy’s focus on foreign affairs over domestic affairs] was the mind-set he brought to the White House, and in some ways this entire [biography of the his life from 1917 to 1956] can be read as an elaborate prolegomenon to Kennedy’s most important foreign policy address, at American University in June 1963, where he urged a realistic reappraisal of the Cold War. (David Kennedy, “Groomed to Be President,” The New York Times, 9/8/2020.)