https://www.mitre.org/ is a world-renowned research organization that aims to help build a safer world. It is probably best known in the information security industry for being the organization behind the industry-standard https://cve.mitre.org/.
These attack vectors are tracked and defined in another well-known knowledge base called https://attack.mitre.org/&, which is also maintained by MITRE. While both the so-called red and blue teams (offensive and defensive security teams) rely on ATT&CK to provide a standardized language around offensive techniques, it does not do the same for defensive techniques.
In other words, it does for the blue team what the ATT&CK framework has done for the red team and offensive techniques since 2013.