prolegomenon
noun
[ ˌprəʊlɪˈɡɒmɪnən ]
• a critical or discursive introduction to a book.
• "a prolegomenon to the second part"
Origin:
mid 17th century: via Latin from Greek, passive present participle (neuter) of prolegein ‘say beforehand’, from pro ‘before’ + legein ‘say’.
