In the context of aviation and aerospace, "air/ground" or "air to ground" refers to the communication or data transmission between an aircraft in the air and a station or system on the ground. This can include various types of communication, such as:

1. Voice communication: Air traffic control (ATC) instructions, clearances, and communications between pilots and controllers.
2. Data transmission: Automatic Dependent Surveillance-Broadcast (ADS-B) data, weather information, flight plans, and other aeronautical data exchanged between the aircraft and ground stations.
3. Navigation and positioning: Signals transmitted from ground stations to aircraft, such as GPS, VOR (VHF Omnidirectional Range), and ILS (Instrument Landing System) signals.

The term "air/ground" or "air to ground" is often used to describe systems, protocols, or interfaces that facilitate this communication, such as:

1. Air/ground data link systems, like ACARS (Aircraft Communication Addressing and Reporting System) or ATN/CPDLC (Aeronautical Telecommunication Network/Controller-Pilot Data Link Communications).
2. Air-to-ground communication networks, like satellite-based systems or cellular networks used for aircraft communication.
3. Air/ground interfaces, such as those used in air traffic management systems, like ADS-B or Multilateration systems.

In general, "air/ground" or "air to ground" refers to the connection between an aircraft and the ground infrastructure, enabling the exchange of information and coordination of activities during flight operations.