It's a very wide term and has many aspects. For example, in a case of the automatic identification of ships, it's when a ship's transponder would send misinformation about its location or its purpose or its cargo or other misinformation.
Another very wide area where it is very pertinent is in positioning, navigation and timing systems such as GPS in the United States or Galileo in Europe, where adversary actions can affect the accuracy of signals and provide completely erroneous information for operational purposes.