DRY: Don't Repeat Yourself
A principle of programming whereby one maintains a separation between aspects of the program, so that changes in one does not impact another.
An example of this would be programs which use introspection of a database to determine what variables, what data types, etc. were available, and would, for instance, use the names from the database as the variables itself, so that one need not have the information coded in multiple places.
See also: