Ypres was a town in Belgium. Some people mispronounced the name as "Wipers". (PROSE: It's a Lovely Day Tomorrow) During World War I, it was the site of a battle between the British and German armies. The British forces used mustard gas during the attack, inflicting heavy casualties on the Germans. (PROSE: Illegal Alien)
Private Daniel Burridge of Prince Albert's (Somerset Light Infantry) Regiment was killed at Ypres on 31 October 1914. He was memorialised in the cemetery in his home village of Perivale. (PROSE: The Hollow Men) Mabel Carter's husband was killed in Ypres, causing her to move back from Shropshire to her old home in London. (PROSE: Pass It On) Amalie Govier also lost her husband Nicolas in Ypres in 1918. (PROSE: Toy Soldiers)
The Twelfth and First Doctors were present at the Christmas conflict in Ypres during the first year of World War I. They used their TARDISes to jump time forward a few hours to ensure a German soldier and Captain Lethbridge-Stewart survived their encounter in a shell hole, and lived to see the Christmas truce. (TV: Twice Upon a Time)
Behind the scenes
There were a total of five battles fought at or near Ypres over the course of the First World War. Doctor Who fiction has yet to make explicit reference to this, although fighting in the area was nearly continuous. The five named battles refer to periods of particularly intense fighting on the salient. The Battle of Passchendaele, referenced in a number of other stories, is also known as the Third Battle of Ypres since the two towns are so close to each other, but this connection is also yet to be made in the DWU.