prolegomenon

Concise Oxford English Dictionary © 2008 Oxford University Press:
prolegomenon /ˌprəʊlɪˈgɒmɪnən/

▶noun (pl. prolegomena) a critical or discursive introduction to a book.

– derivatives
prolegomenous adjective.

– origin C17: via L. from Gk, passive pres. part. (neut.) of prolegein ‘say beforehand’, from pro ‘before’ + legein ‘say’.