preamble
    n 1: a preliminary introduction to a statute or constitution
         (usually explaining its purpose)
    v 1: make a preliminary introduction, usually to a formal
         document

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.
   [1913 Webster]

Preamble \Pre"am`ble\, v. t. & i.
   To make a preamble to; to preface; to serve as a preamble.
   [R.] --Feltham. Milton.
   [1913 Webster]

PREAMBLE. A preface, an introduction or explanation of what is to follow:  
that clause at the head of acts of congress or other legislatures which 
explains the reasons why the act is made. Preambles are also frequently put 
in contracts to, explain the motives of the contracting parties, 
     2. A preamble is said to be the key of a statute, to open the minds of 
the makers as to the mischiefs which are to be remedied, and the objects 
which are to be accomplished by the provisions of the statutes. It cannot 
amount, by implication, to enlarge what is expressly given. 1 Story on 
Const. B 3, c. 6. How far a preamble is to be considered evidence of the 
facts it recites, see 4 M. & S. 532; 1 Phil. Ev. 239; 2 Russ. on Cr. 720; 
and see, generally, Ersk. L. of Scotl. 1, 1, 18; Toull. liv. 3, n. 318; 2 
Supp. to Ves. jr. 239; 4 L. R. 55; Barr. on the Stat. 353, 370.