What is define null?

In computer programming, null is a special value used to denote the absence of any value or a reference to a non-existent object or entity. It can be considered as a placeholder for a missing value in a variable or a pointer that is not pointing to any memory location.

Null can be represented in different ways in different programming languages, such as NULL in C and C++, None in Python, nil in Ruby and Objective-C, and undefined in JavaScript.

Null can also be used to indicate the result of an operation that failed to produce a meaningful result, such as a database query that returns no records or a function that encounters an error.

In summary, null is a reserved keyword in programming languages that has a special meaning to indicate the absence of a value or a reference.