Official Aviation Definition
A NOTAM (Notice to Air Missions, formerly Notice to Airmen) is an official notice containing information concerning the establishment, condition, or change in any aeronautical facility, service, procedure, or hazard, the timely knowledge of which is essential to personnel concerned with flight operations.
In Simple Words
A bulletin sent to pilots warning them about broken navigation equipment, closed runways, or construction cranes near airports.
Real-World Flight Example
A NOTAM alerted crews that runway 04R/22L was closed for resurfacing between 2200Z and 0600Z.