For what does A.C. stand on French wine bottles?

On a French wine bottle, A.C. stands for 'Appellation Contrôlée'. It is sometimes written as A.O.C., meaning 'Appellation d’Origine Contrôlée'. In French, the word 'appellation' refers to the designation or geographical region, and 'Contrôlée' means "controlled". Therefore, 'Appellation Contrôlée' means "controlled designation. (Likewise, 'Appellation d’Origine Contrôlée' means "controlled designation of origin".)

Each region of France has different laws for winemaking. These laws control the physical boundaries of vineyards, the permitted grape varieties, pruning and planting techniques, ageing methods and so forth. Often, a wine bottle may state a location before the letters A.C. to indicate where the wine came from, for example, 'Bordeaux AC' indicates the wine came from the region of Bordeaux.

Since Bordeaux is a rather large region, laws may differ slightly between vineyards. In these instances, the A.C. label may be more specific. 'Pauillac AC', for example, indicates the wine was made in a sub-region of Haut-Médoc, which itself is a smaller region of the Bordeaux viticultural area.