What is Sterling Silver?
Sterling Silver is at least 92.5% pure silver. If it is over 7.78gms in weight and sold in the UK then it should have a UK Hallmark indicating that percentage, where it was hallmarked and the maker. This is the minimum required marks but older items will also have a date letter, but this is now optional. This means the item is all silver not just "plated" with a thin layer of silver over base metal. The Hallmark is a guarantee of this as it can only be legally marked at an official Assay Office who assay (test) the item for purity. Silver commonly has a stamp 925 somewhere on the item. A 925 mark or stamp can be applied by anyone and is not a guarantee of anything. Other countries have different rules and controls so items purchased from those countries may not have the same guarantee of quality.
These images show a 925 stamp and a traditional hallmark. The Hallmark proves that the item is Sterling Silver as indicated by the Lion and the 925. The Leopard face indicates it was marked in the London Assay Office and the S shows that it was marked in 2017. The MB is Madeline's registered maker or sponsors mark. Each maker (or sponsor) has a stamp design that is registered at the Assay Office to prove that it is unique to them. Traceability may be a modern buzzword but this system goes back hundreds of years.
The origin of the term "Sterling" is debated by historians and is probably lost in the "mists of time".
Although the hallmark was traditionally stamped into the item leaving an impression or stamp. In recent years technology has enabled the Assay Office to cut the mark with a laser to leave generally a shallower impression than a stamp.
EPNS silver is an acronym for electro-plated nickel silver. Although it has a mark or stamp that looks similar to a hallmark it is only silver plated and not "Sterling".