https://arstechnica.com/information-technology/2023/01/an-aviation-expert-explains-how-the-faas-critical-notam-safety-system-works/

Late in the evening of Jan. 10, 2023, an important digital system known as NOTAM run by the Federal Aviation Administration went offline. The FAA was able to continue getting necessary information to pilots overnight using a phone-based backup, but the stopgap couldn’t keep up with the morning rush of flights, and on Jan. 11, 2022, the FAA grounded all commercial flights in the U.S. In total, nearly 7,000 flights were canceled. Brian Strzempkowksi is the interim director of the Center for Aviation Studies at The Ohio State University and a commercial pilot, flight instructor and dispatcher. He explains what the NOTAM system is and why planes can’t fly if the system goes down.

Aviation is full of acronyms, and Notice to Air Missions, or NOTAM, is one acronym that pilots learn early on in their training. A NOTAM is quite simply a message that is disseminated to flight crews of every aircraft in the US.