Hiroshima was a city in Japan. It was hit by an atomic bomb in 1945, thus causing the Japanese to surrender, ending the Pacific War and the Second World War as a whole. (COMIC: Sky Jacks)
In 1996, Chad Vandemeer used the GCI processor to create the Through the Eye of Eternity episode "Miss Hiroshima". It was subsequently lost in the archives through possible interference from the enemy. (PROSE: The Book of the War)
The Seventh Doctor listed the bombing of Hiroshima as one of the terrible things that happened after he set history back on track by freeing the Timewyrm from Adolf Hitler's mind. (PROSE: Timewyrm: Exodus)