The NOT operator is a fundamental logical operator used in programming and mathematics. It negates a given condition or statement, meaning if the condition is true, the NOT operator makes it false, and vice versa. For example, if a statement is "It is raining," applying the NOT operator would change it to "It is not raining." In computer science, the NOT operator is often represented by the symbol "!" or the word "NOT." It is commonly used in conditional statements and boolean logic, which are essential for decision-making processes in programming languages like Python and Java.