NOAA's Space Weather Prediction Center

Space weather refers to variations in the space environment between the sun and Earth (and throughout the solar system) that can affect technologies in space and on Earth. In addition to the heat and
light we are familiar with, the sun produces coronal mass ejections,
flares, particle events, and high speed solar wind streams. When
these phenomena are aimed at Earth, we get space weather.
Producing accurate space weather forecasts depends on understanding these phenomena and how they interact with the near-Earth environment.