"Médoc" means "the middle land." North of the city of Bordeaux, travelling up alongside the Gironde until we reach the Atlantic, is a region known as the Médoc. This is not an appellation, or a commune, but a geographical region that encompasses Margaux, St Julien, Pauillac and St Estèphe, as well as all those vineyards scattered to the west of these famous appellations, and also those to the north, beyond the boundary of the St Estèphe commune.
Thus, when someone talks of the wines of the Médoc, there is a good chance that they are referring to this region as a whole. When spotted on a wine label, however, the word Médoc means something very different; when used as an appellation, it refers to a much smaller, well defined and discreet region of Bordeaux, north of St Estèphe.
Estates in the four aforementioned communes, as well as those located to the west of these, inland of the famous gravel croupes that bear the greatest vineyards of the left bank, are not entitled to the appellation. The same is also true of those estates that lie south of the 'big four', in the hinterland between the vineyards of Margaux and the suburbs of Bordeaux. All these vineyards have an entitlement to the Haut-Médoc appellation, one which has perhaps a touch more cachet than mere Médoc. Source: Wine Doctor