Space weather is generally described as events and disturbances originating at the Sun that propagate and interact with geospace to produce impacts on humans, signals and engineered systems. 
A slightly broader definition additionally considers:

  1. Disturbances and variations in Earth’s space-atmosphere interface region that may propagate upward from Earth’s surface and lower atmosphere;
  2. Influences of cosmic rays on humans, hardware and Earth’s atmosphere; and
  3. Similar disturbances at other planets that humans or their hardware may visit.   

​This general definition covers routine and extreme behavior.

From a national security perspective, the US National Space Weather Action Plan (2015)  notes space-weather events as naturally occurring phenomena that have the potential to disrupt electric power systems; satellite, aircraft, and spacecraft operations; telecommunications; position, navigation, and timing services; and other technologies and infrastructures that contribute to the Nation’s security and economic vitality. 

More recently, the US National Space Weather Strategy And Action Plan (2019)  defined space weather as comprising a set of naturally occurring phenomena that have the potential to adversely affect critical functions, assets, and operations in space and on Earth. Extreme space weather events can degrade or damage critical infrastructures, which may result in direct or cascading failures across key services such as electric power, communications, water supply, healthcare, and transportation.