What is the definition of a "null string"?

A "null string" is defined as a string of length 0. This means that it is a string that contains no characters at all, and it can be represented in programming as either "" (an empty string) or sometimes as null in certain languages, though the terms can vary slightly in their usage.

The significance of a null string is that it can represent the absence of any value or a lack of content in situations where a string is expected. In contrast, other choices refer to strings that have specific content, such as whitespace, special characters, or error messages, all of which imply that there are characters present in the string. Therefore, the option stating that a null string is a string of length 0 accurately embodies the concept as it pertains to computer science.