That will depend on the stamp and the country it is from. Sometimes it is the engraver's name. It could be a plate number. It might even be the name of the printing company.
It is Italian for a postage stamp.
It allows for International Posting of Business letters without applying a postage stamp in country of posting.
It means one individual stamp.
Ajman
"Sverige" is the Swedish word for Sweden. On a postage stamp, it indicates that the stamp is from Sweden or related to the country in some way.
Austrian Republic
It is most likely the stamp of the manufacturer
50th anniversary of the death of Franz Lehar
Yes, if you mean the Scott Standard Postage Stamp Catalogue ,each its 6 volumes can bought separately as can the US specialized catalogue.
The British Penny Black stamp was the world's first adhesive postage stamp and it went on sale in Great Britain on May 6th 1840.
It means "stamp" - the stamps which are pasted on letters when you send them.
The paper labels on mailed papers are called postage stamps. They indicate that postage has been paid for the delivery of the mail.