“A.C.” is an abbreviation with several common meanings depending on context:

• Air conditioning
  • Example: Turn on the A.C. = cooling system in a room or vehicle.
• Alternating current (AC)
  • In physics/electrical engineering: electric current that periodically reverses direction.
• Anno Christi (less common / historical usage)
  • Latin-based dating meaning “in the year of Christ,” similar to A.D.
• After Christ (rare, informal/incorrect usage)
  • Sometimes mistakenly used instead of A.D.

In short, A.C. most commonly means “air conditioning” or “alternating current,” depending on the context.