Log4j vulnerability (noun)
An open source Java-based software tool available from the Apache Software Foundation designed to log security and performance information.
CyberWire Glossary link
Audio reference link: “CISA Director: The LOG4J Security Flaw Is the ‘Most Serious’ She’s Seen in Her Career,” by Eamon Javers (CNBC) and Jen Easterly (Cybersecurity and Infrastructure Security Director) YouTube, 20 December 20 2021.