Step 1: Understand the concept of functions.
A function is a block of reusable code designed to perform a particular task. Functions help in organizing programs and reducing code repetition.
Step 2: Define user defined function.
A user defined function is a function that is written and defined by the programmer rather than being provided by the programming language.
These functions allow programmers to create custom operations and reusable program components.
Step 3: Example in Python.
\[
\texttt{def add(a,b):}
\]
\[
\texttt{\ \ return a+b}
\]
Here the programmer defines a function named add which returns the sum of two numbers.
Step 4: Conclusion.
Thus a user defined function is a programmer-created function used to perform specific tasks in a program.