It often happens that it is necessary to alter a stamp at short notice-- to raise or lower its face value, to make it available for use in another country or otherwise to change its original inscriptions. It is then that, as a temporary measure, stamps are altered by having the new inscription or value printed over them.
Anything added to the stamp in this way after it is completed is called on overprint. Stamp collectors have come to apply this term to alterations or additions which do not affect the face value of the stamp, any addition which alters or confirms the value is called a surcharge.
The first issue of stamps of Gibraltar was made by overprinting stamps of Bermuda with the word "GIBRALTAR," these stamps being used while a proper set of stamps for "the Rock" was being prepared.
As an example of a surcharge which alters the value of a stamp, the 1/4 on half-anna stamps of India may be instanced.
Sometimes overprints will indicate a change in the status of a country, e.g. the letters "V.R.£ on early stamps of Fiji, when it was ceded to Great Britain. Sometimes they are a kind of advertisement calling attention to a big exhibition or an important anniversary in a country's history.
What their purport, they always some message for the collector, and they should be carefully noted.
There are one or two instances where overprinting has been resorted to in order to change the design of a stamp. The arms device on the stamps of Salvador, issued on January 1st, 1895, is an overprint which blots out the portrait of a president whew had been deposed during the time when the stamps were being prepared, while there is an issue of Peru which bears the overprinted portrait of a General Bermudez.