3-letter code that represents a set of locale settings for a specific country, region or carrier.
A CSC is a 3-letter code that representes a set of locale settings for a specific country, region, or even carrier. The settings include software packages specific to the region, carrier branding and APN settings for data connection, MMS and SMS. Additionally a CSC may determine which apps are available to you. Apps can also use the CSC to determine your region and serve you relevant regional news.
Some say that CSC stands for 'Country (or Carrier) Specific Code' but officially it stands for Consumer Software Customisation code.
A CSC is not necessarily bound to the country where you bought the phone and it is possible to change a CSC to a completely different one, but after changing it your phone may need to reboot and may remove and reinstall all apps.
For example, the code XEU refers to Great Britain, and VDI refers to the carrier Vodafone in Ireland.
Note the difference between Consumer Software Customisation codes and Common Short Codes, a different concept with the same abbreviation.
On Samsung Phones you can dial *#1234# to check the set CSC (more info here)