This is when Royal Mail officially reprints a stamp that is out of use. 2005 is the most recent example of this. Royal Mail are re-issuing the popular Castles set of stamps from 1955, which featured Edinburgh, Windsor, Carrickfergus and Caernarfon castles. The values in the set at the time were 2/6d, 5/, 10/- and £1. To commemorate the 50th anniversary of these Castles high values, Royal Mail is issuing a miniature sheet on 22nd March 2005 which features the 1955 designs. The sheet has been brought up to speed with the decimal age by including two 50p stamps (Carrickfergus and Windsor) and two £1 stamps (Edinburgh and Caernarfon). Similar miniature sheets were issued in December 2000 and May 2003 to mark the 50th Anniversary of the issue of the Wilding low value definitives.
You can see the re-issued Castles miniature sheet on our cover illustrated here.