Data that standardizes the choices available to users for a data field or that provides common standards for a data item is referred to as reference data. For example, when you are asked to enter your address in a web form, you might be given a drop-down that lets you choose from a list of states or countries. The data in that drop-down is reference data. Reference data seems very straightforward but turns out to have subtle challenges. For example, consider the list of countries that are available to choose from. The country list you can choose from when entering your current address might be different from the country list you see when asked to enter the country in which you were born, as some countries might have ceased to exist or changed their names (perhaps Macau, for example). These types of issues can lead to many unexpected complexities in an IT system.
— War and Peace and IT: Glossary, Mark Schwartz