What's the difference between a forgery and reproduction?

It's a fine line, and not everybody follows the same definition. But, in my neck of the woods, generally...

counterfeit -- fake produced to defraud the postal service
forgery -- fake produced to defraud collectors (often, but not always, the design is not perfectly true to original design)

Counterfeits are forgeries, but forgeries are not necessarily counterfeits.

reproduction -- made from either original plates, or a copy made from (and true) to original design
-- reproductions made be made by the postal authorities or privately
-- usually they are not on the exact same paper stock as the originals, as they were usually made years after last print run
-- the colors/perforations/scaling... may also be different, but the design is typically identical
-- some reproductions are made on different material, such as metal
-- some reproductions lack the design details or are made by a different printing process ("gold" replicas are an example)
-- reproductions are sold as reproductions, with no intent to defraud by the original seller

Greetings Fellow SB Boarders
.
I had to dig out my "The Stamp Collector's Encyclopaedia" by R.J. Sutton for some definitions:
.
"Counterfeit Stamps are those that have been unofficially and fraudulently imited and copied, with a view to their being passed off as genuine. They are usually referred to as forgeries.
.
Forgery. A fraudulent imitation of a genuine stamp. There are two principal types: the postal forgery, made to deceive the Post Office, and the philatelic forgery, made to deceive collectors. For postal forgeries, especially used on cover, there is a keen demand from specialists. A special type of postal forgery is the propaganda forgery, made to be smuggled into enemy territory and there used by secret agents or Resistance workers. Stamps of enemy or enemy-occupied countries were forged by the authorities in Britain in both the First and Second World Wars, though in the former case it seems unlikely that they were used for the purpose for which they were intended. Other items sometimes described as propaganda forgeries are imitations of enemy stamps never intended for postal use but in which the design is distorted for propaganda purposes; for example stamps in which Hitler's head appeared as a skull. These later are better described as propaganda labels.
.
Philatelic forgeries of rare overprints or rare postmarks have sometimes been applied to perfectly genuine stamps; in such cases the complete item may be termed a fake.
.
Fake. A genuine stamp which has been altered or repaired in some way to make it appear what it is not, for the purpose of deceiving collectors. Stamps may be faked either to hide a defect or to make a common item resemble a rare one. Great ingenuity has been used on fakes; stamps which were perforated have been made to appear imperforate, and vice versa; used stamps have had their postmarks removed and the backs regummed to make them appear mint; and so on.
.
Replicas. In the early days of stamp collecting, printed sheets of stamps reproductions were obtainable as 'space fillers' to enable a collector to show at least a replica of those specimens not yet obtained, or - often - unobtainable. These 'stamps' were all printed in one colour (blue was usual), were approximately of correct dimensions, and in many instances were very close copies of the original stamps, and are thus often wrongly denounced as forgeries. They still turn up from time to time in old collections.
.
Reprints. Stamps printed from plates after official issues have ceased, for official files, as philatelic curiosities, as official souvenirs, or for sale to meet a collector demand. When done officially, such reprints are often in new colours, or are imperf. or have some other distinction so that the initiate is not deceived, but there have been many reprints done with an ulterior motive. There is a convusing modern tendency to apply the name to fresh printing of current stamps from existing plates to renew stocks."
.
Thus sayeth Mr. Sutton.
.
Respectfully,

Gordon Lee

PS.  There is no definition for "Reproduction."