Pertaining to, or of the nature of, a preface; introductory to a book, essay, or discourse

ETYM Pref. pre + Latin liminaris belonging to a threshold, from limen, liminis, threshold, entrance: cf. French préliminaire. Related to Limit.
Designed to orient or acquaint with the a situation before proceeding.

ETYM Latin praevius going before, leading the way; prae before + via the way. Related to Voyage.
1. Just preceding something else in time or order; SYN. old.
2. Too soon or too hasty; SYN. premature.

ETYM Latin prior former, previous, better, superior; compar. corresponding to primus first, and pro for. Related to Former, Prime, and Pre-, Pro-.
1. Earlier in time or order; previous.
2. Taking precedence (as in importance).
3. Former; antecedent; anterior; previous.