Real Dictionary

preface
countable noun: A preface is an introduction at the beginning of a book, which explains what the book is about or why it was written.
verb: If you preface an action or speech with something else, you do or say this other thing first.

preface in American English
the introduction to the Canon of the Mass, ending with the Sanctus
to furnish or introduce with a preface
noun: a preliminary statement in a book by the book's author or editor, setting forth its purpose and scope, expressing acknowledgment of assistance from others, etc.