A user-defined function is a code created by a user that prompts the performance of an action and the computation of a value. The function can be programmed to activate via a designated keystroke. There are as many possible uses for user-defined function keys as there are keystrokes. An example of an user defined function is the programming of keys to trigger the activation or disabling of sound.

User-defined functions are subroutines, of which there are several types. An external function can be written using many operating languages, including Java. An SQL function is written entirely in SQL. A sourced function compliments and modifies an existing user-defined function.

In a spreadsheet program, a user-defined function can be applied to a cell, a region, defined by a column number and a row number, to which data is added. The function will occur when the data is entered. A user-defined function cannot alter the spreadsheet environment, such as by the selection, deletion, insertion, renaming, or formatting of existing data. The user-defined functions are the only function procedures that the Excel spreadsheet can use in a cell's formula.

Typically, a user-defined function manipulates or calculates a result based on the data, or on the arguments, that were specified as part of the defining of the function. The structure of a user-defined function, for a spreadsheet, consists of beginning and ending identifiers (Function and End Function, respectively) with so-called arguments forming the bulk of the function definition. Arguments are the qualifiers to the function, with criteria used in analyzing the data, and the order or hierarchy of analyses, as examples.