Passchendaele is the French name of the village, and it's not the only Belgian town that the British army called by the French rather than the Flemish name. Passchendale was fought over in the 3rd Battle of Ypres (not Ieper) and another part of the battle involved the Messines (not Mesen) ridge. Whether this is due to the army using French maps, or whether French was the more prestigious language in Belgium itself at the time I'm not sure.

As /u/lu619 noted below, 'Passchendaele' is the French rendering of the Flemish 'Passendaele', which refers to a town northeast of Ypres, Belgium; the name itself means 'Way of thru Passion', referring to the route taken by Jesus Christ to his death in the Cross at Golgotha. It is located at the eastern end of a high ground complex comprising the Messines-Wyschaete ridge and the Passendaele-Staden Ridge, connected together by the Gheluvelt Plateau. From June to November 1917, these areas of high ground were the focus of the British Ypres Offensive, referred to as the 3rd Battle of Ypres.

The proper terminology for the campaign in question is the '3rd Battle of Ypres' or '3rd Ypres Campaign'; I avoid 'Passchendaele', as this name has become loaded with baggage since the 1930s, most of which has obscured the campaign itself.