Free space is commonly used to refer to EM propagation in air in RF contexts. For example: https://en.wikipedia.org/wiki/Free-space_path_loss https://en.wikiped

Free space is commonly used to refer to EM propagation in air in RF contexts. For example: https://en.wikipedia.org/wiki/Free-space_path_loss https://en.wikipedia.org/wiki/Free-space_path_loss Here the distinction is between propagation models that include ground plain, terrain, or airborne clutter.