Common Terms and Abbreviations

bogus stamp - a technical term for an "in-house" stamp issued locally as a souvenir and/or for local political/propaganda purposes, with no validity for carriage in the postal system. As opposed to a 
counterfeit stamp which is designed to defraud.
CAPEX Canadian Philatelic Exhibition (held 1951, 1978, 1987, 1996) 
FDC First Day Cover
MS Miniature Sheet
not listed - either "in excess of postal needs", or "not available to the public in reasonable quantities"
not recognized - not recognized by the UPU as a postal authority - also see "bogus stamp" .
O Official
OHMS On His/Her Majesty's Service
P parcel post/stamp
PC postcard
...PEX Philatelic Exhibition
PPE Prepaid envelope
PPPC Prepaid postcard
RPO Railway Post Office
railway letter (stamp) - a stamp issued by a tourist/heritage/preservation railway for carriage of a letter within its domain  
revenue stamp - a stamp issued for the purpose of collecting a fee other than for carriage of mail 
UIC Union Internationale des Chemins de Fer (founded 1922)
UPU Universal Postal Union (established 1874).