What does EPNS mean on silver items?

Although my interest primarily relates to pieces made of sterling, I receive many questions about silver plate items. Perhaps the most common is, "What does 'EPNS' mean?"
The answer is that it's an indicator that a piece is made of silver plate. Other similar terms are:

• EPNS
• EP
• EP over copper
• EPBM
• EPCA
• EPGS
• EPWM
• EPNS A1
• EPNS England
• EPNS Sheffield
• EPNS X1

See this article for a bit more information: What does 'EPNS' mean on my silver pieces?