Source : Webster's Revised Unabridged Dictionary (1913)

Preamble \Pre"am`ble\, n. [LL. praeambulum, from L. praeambulus
   walking before, fr. praeambulare to walk before; prae before
   + ambulare to walk: cf. F. pr['e]ambule. See {Amble}.]
   A introductory portion; an introduction or preface, as to a
   book, document, etc.; specifically, the introductory part of
   a statute, which states the reasons and intent of the law.

Preamble \Pre"am`ble\, v. t. & i.
   To make a preamble to; to preface; to serve as a preamble.
   [R.] --Feltham. Milton.

Source : WordNet®

preamble
     n : a preliminary introduction to a statute or constitution
         (usually explaining its purpose)
     v : make a preliminary introduction, usually to a formal
         document