“A.D.C.” is an abbreviation with a few meanings depending on context:

• Aide-de-camp — a military or official assistant to a high-ranking officer or head of state (the most common meaning).
  • Example: a general’s personal assistant or staff officer.

Other meanings:

• Analog-to-Digital Converter (ADC) — in electronics, a device that converts analog signals (like sound or voltage) into digital data.
• Air Defense Command (historical military use in some countries)

In short, A.D.C. most commonly means “aide-de-camp” in military/government contexts, or “analog-to-digital converter” in electronics.