The Collaborative International Dictionary of English v.0.48:

Postage \Post"age\, n.
   The price established by law to be paid for the conveyance of
   a letter or other mailable matter by a public post.
   [1913 Webster]

   Postage stamp, a government stamp required to be put upon
      articles sent by mail in payment of the postage, esp. an
      adhesive stamp issued and sold for that purpose.
      [1913 Webster]

WordNet (r) 3.0 (2006):

postage
    n 1: the charge for mailing something
    2: a small adhesive token stuck on a letter or package to
       indicate that that postal fees have been paid [syn:
       postage, postage stamp, stamp]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

POSTAGE. The money charged by law for carrying letters, packets and
documents by mail. By act of congress of March 3, 1851, Minot's Statute at
Large, U. S. 587, it is enacted as follows:
     2.-Sec. 1. That from and after the thirtieth day of June, eighteen
hundred and fifty-one, in lieu of the rates of postage now established by
law, there shall be charged the following rates, to with or every single
letter in manuscript, or paper of any kind, upon which information shall be
asked for, or communicated, in writing, or, by marks or signs, conveyed in
the mail for any distance between places within the United State's, not
exceeding three thousand miles, when the postage upon such letter shall have
been prepaid, three cents, and five cents when the postage thereon shall not
have been prepaid; and for any distance exceeding three thousand miles,
double those rates. For every such, single letter or paper when conveyed
wholly or in part by sea, and to or from a foreign country, for any distance
over twenty-five hundred miles, twenty cents, and for any distance under
twenty-five hundred miles, ten cents, (excepting, however, all cases where
such postages have been or shall be adjusted at different rates, by postal
treaty or convention already concluded or hereafter to be made;) and for a
double letter there shall be charged double the rates above specified; and
for a treble letter, treble those rates; and for a quadruple letter,
quadruple those rates; and every letter or parcel not exceeding half an
ounce in weight shall be deemed a single letter, and every additional weight
of half an ounce, or additional weight of less than half an ounce, shall be
charged with an additional single postage. And all drop letters, or letters
placed in any post office, not for transmission, but for delivery only,
shall be charged with postage at the rate of one cent each; and all letters
which shall hereafter be advertised as remaining over or uncalled for in any
post office, shall be charged with one cent in addition to the regular
postage, both to be accounted for as other postages are.
     3.-Sec. 2. That all newspapers not exceeding three ounces in weight,
sent from the office of publication to actual and bona fide subscribers,
shall be charged with postage as follows, to wit: All newspapers published
weekly only, shall circulate in the mail free of postage within the county
where published, and that the postage on the regular numbers of a newspaper
published weekly, for any distance not exceeding fifty miles out of the
county where published, shall be five cents per quarter; for any distance
exceeding fifty miles and not exceeding three hundred miles, ten cents per
quarter; for any distance exceeding three hundred miles and not exceeding
one thousand miles, fifteen cents per quarter; for any distance exceeding
one thousand miles and not exceeding two thousand miles, twenty cents per
quarter; for any distance exceeding two thousand miles and not exceeding
four thousand miles, twenty-five cents per quarter; for any distance
exceeding four thousand miles, thirty cents per quarter; and all newspapers
published monthly, and sent to actual and bona fide subscribers, shall be
charged with one-fourth the foregoing rates; and on all such newspapers
published semi-monthly shall be charged with one-half the foregoing rates;
and papers published semi-weekly shall be charged double those rates;
triweekly, treble those rates; and oftener than tri-weekly, five times,
those rates. And there shall be charged upon every other newspaper, and each
circular not sealed, handbill, engraving, pamphlet, periodical, magazine,
book, and every other description of printed matter, which shall be
unconnected with any manuscript or written matter, and which it may be
lawful to transmit through the mail, of no greater weight than one ounce,
for any distance not exceeding five hundred miles, one cent; and for each
additional ounce or fraction of an ounce, one cent; for any distance
exceeding five hundred miles and not exceeding one thousand five hundred
miles, double those rates; for any distance, exceeding one thousand five
hundred miles and not exceeding two thousand five hundred miles, treble
those rates; for any distance exceeding two thousand five hundred miles and
not exceeding three thousand five hundred miles, four times those rates; for
any distance exceeding three thousand five hundred miles, five times those
rates. Subscribers to all periodicals shall be required to pay one quarter's
postage in advance, and in all such cases the postage shall be one-half the
foregoing rates. Bound books, and parcels of printed matter not weighing
over thirty-two ounces, shall be deemed mailable matter under the provisions
of this section. And the postage on all printed matter other than newspapers
and periodicals published at intervals not exceeding three months, and sent
from the office of publication, to actual and bona fide subscribers, to be
prepaid; and in ascertaining the weight of newspapers for the purpose of
determining the amount of postage chargeable thereon, they shall be weighed
when in a dry state, And whenever any printed matter on which the postage is
required by this section to be prepaid, shall, through the inattention of
postmasters or otherwise, be sent without prepayment, the same shall be
charged with double the amount of postage which would have been chargeable
thereon if the postage had been prepaid; but nothing in this act contained
shall subject to postage any matter which is exempted from the payment of
postage by any existing law, And the postmaster general, by and with the
advice and consent of the president of the United States, shall be, and he
hereby is, authorized to reduce or enlarge, from time to time, the rates of
postage upon all letters. and other mailable matter conveyed between the
United States and any foreign country for the purpose of making better
postal arrangements with other governments, or counteracting any adverse
measures affecting our postal intercourse with foreign countries, and
postmasters at the office of delivery are hereby authorized, and it shall be
their duty, to remove the wrappers and envelopes from all printed matter and
pamphlets not charged with letter postage, for the purpose of ascertaining
whether there is upon or connected with any such printed matter, or in such
package, any matter or thing which would authorize or require the charge of
a higher rate of postage thereon. And all publishers of pamphlets,
periodicals, magazines, and newspapers, which shall not exceed sixteen
ounces in weight, shall be allowed. to interchange their publications
reciprocally, free of postage: Provided, That such interchange shall be
confined to a single copy of each publication: And provided, also, That said
publishers may enclose in their publications the bills for subscriptions
thereto, without any additional charge for postage; And provided, further,
Thai in all cases where newspapers shall not contain over three hundred
square inches, they may be transmitted through the mails by the publishers
to bona fide subscribers, at one-fourth the rates fixed by this act.
     5. By the act of March 3, 1845, providing for the transportation of the
mail between the United States and foreign countries, it is enacted by the
3d section, that the rates of postage to be charged and collected on all
letters, packages, newspapers, and pamphlets, or other printed matter,
between the ports of the United States and the ports of foreign governments
enumerated herein, transported in the United States mail under the
provisions of this act, shall be as follows: Upon all letters and packages
not exceeding one-half ounce in weight, between any of the ports of the
United States and the ports of England or France, or any other foreign port
not less than three thousand miles distant twenty-four cents, with the
inland postage of the United States added when sent through the United
States mail to or from the post office at a port of the United States; upon
letters and packets over one-half an ounce in weight, and not exceeding one
ounce, forty-eight cents; and for every additional half ounce or fraction of
an ounce, fifteen cents; upon all letters and packets not, exceeding one-
half ounce, gent through the United States mail between the ports of the
United States and any of the West India islands, or islands in the Gulf of
Mexico, ten cents; and twenty cents upon letters and packets not exceeding
one ounce; and five cents for every additional half ounce or fraction of an
ounce; upon each newspaper, pamphlet, and price current, sent in the mail
between the United States and any of the ports and places above enumerated,
three cents, with inland United States postage added when the same is
transported to or from said port of the United States in the United States
mail.