A testing technique that sends various inputs to a interface to study how it reacts.
Examples
Buffer overflow
Input strings of increasing lengths to see when the binary breaks.
SQL injections
Input special characters and seeing how the server reacts.
Finding hidden pages on web applications
Send HTTP requests prepending or appending common names (from a wordlist) to the URL to see if the web applications responds with 200 OK, meaning that the sub-domain or directory is publicly available.