England and Wales
England and Wales are two separate countries within the United Kingdom. However, for many administrative purposes they are treated as the single entity England and Wales. Specifically, the two countries share the same legal system, the successor to that of the Kingdom of England, and as a consequence, most laws.
The other parts of the United Kingdom, that is Scotland and Northern Ireland, often have laws very different from those of England and Wales. Scotland in particular has an entirely independent court system, and it is more frequent to have legislation for England, Wales and Northern Ireland than for England, Wales and Scotland.
As another example, in the sport of cricket, England and Wales field a single representative team in international competition, whereas Scotland is treated as a separate entity. The England and Wales team (often abbreviated simply as England) is administered by the England and Wales Cricket Board.
From the 1536 Act of Union until much later, Wales was treated as part of England, so this formula was not necessary. For example, the 1707 Act of Union is referred to as the union of England and Scotland - under modern terms this would be the union of Scotland with England and Wales. Sometimes the modern term is used anachronistically to refer to the past.