Some programmer thinks that defining a function inside an another function is known as “nested function”. But the reality is that it is not a nested function, it is treated as lexical scoping. Lexical scoping is not valid in C because the compiler cant reach/find the correct memory location of the inner function.

In C programming, a nested function is a function defined inside another function. The nested function has access to the variables and parameters of the outer function, which is known as the enclosing function.

When a function is defined inside another function, it can be accessed only from within the enclosing function, and it cannot be called from outside. The nested function can also access variables and parameters of the enclosing function, but not the variables and parameters of other functions.

The syntax for defining a nested function in C is as follows: 

void outer_function() {
   // Enclosing function
   void nested_function() {
      // Nested function
   }
}

Nested functions can be useful in situations where a small, self-contained function is needed only within a larger function. By defining the nested function within the enclosing function, the code can be organized and the scope of the nested function can be limited to the enclosing function.

However, it's important to note that nested functions are not a part of the C language standard and are not supported by all compilers. Additionally, their use is generally discouraged in modern C programming, as there are other ways to achieve the same functionality with better code organization and readability.