Plain English Meaning
A NOTAM is an official short-notice alert telling pilots about something that has changed at an airport, along a route, or in the airspace — closed runways, broken lights, restricted areas, and so on — so they can plan around it before they fly.
Full Definition
A NOTAM is a time-critical notice issued by aviation authorities to alert pilots and other flight personnel of conditions affecting the National Airspace System that are not known far enough in advance to be published on charts or in standard flight publications. NOTAMs cover items such as runway closures, outages, construction, airspace , , unlighted , and changes to or . Pilots are required to relevant NOTAMs as part of under 91.103.