The term "shell shock" refers to a condition that VA mental health professionals now call PTSD. The term first came about during WWI and its aftermath when soldiers exhibited symptoms such as uncontrollable shaking, fugues, and panic induced by loud noises. After intense periods of enemy shelling, a spike rose in soldiers reporting these symptoms leading health officials to use the term "shell shock." Doctors blamed the concussive blasts from the shelling for the symptoms and believed it was a physical problem, overcome by simply resting.
As the science of mental health improved over the century, doctors recognized the symptoms as a mental disorder that could be controlled with proper care.
Sources:
Wade Davis, Into the Silence the Great War, Mallory, and the Conquest of Everest (New York: Alfred A. Knopf, 2011),
Adam Hochschild, To End All Wars: A Story of Loyalty and Rebellion, 1914-1918 (Boston: Houghton Mifflin Harcourt, 2011).
Barbara Wertheim Tuchman, The Guns of August (New York: Ballantine, 1994).
Richard J Evans, The Coming of the Third Reich (New York: The Penguin Press, 2009).
Depends what you mean under "shell shock" - at first, it has very narrow meaning and got applied more frivolously over time.
Originally, massive artillery barrages of WWI caused typical symptoms. Later other causes led to same symptoms - aerial bombardments, intensive machine gun fire, gas attacks, but results still were called "shell shock". Classical symptoms is what I'd call a "narrow definition".
During the early stages of World War I, soldiers from the British Expeditionary Force began to report medical symptoms after combat, including tinnitus, amnesia, headache, dizziness, tremor, and hypersensitivity to noise. While these symptoms resembled those that would be expected after a physical wound to the brain, many of those reporting sick showed no signs of head wounds. By December 1914 as many as 10% of British officers and 4% of enlisted men were suffering from "nervous and mental shock"
Also note, that shell shock wording was banned by British war-time censors and postconcussional syndrome was used instead - even in medical journals.
After WWI, "shell shock" was gradually expanded to all psychological traumas, related to the Great War and to other wars as well - for the lack of better terms. That's what I would call "wide definition".
In its original, "narrow" definition, I believe it could be classified as "extreme case of PTSD" in modern speak. In wider definition I see it as equal to PTSD (which is quite nebulous collection of very different psychiatric ailments, bunched together by their cause only).
During WWII, shell shock was replaced with combat stress reaction - which better describes the phenomenon. And nowadays we arrived to PTSD - as you see, combat is gone completely, as same symptoms could be found in purely civilian population too.
Sources
PDF - Jones, E, Fear, N and Wessely, S. "Shell Shock and Mild Traumatic Brain Injury: A Historical Review". American Journal of Psychiatry 2007; 164:1641–1645
In the early years of World War One, shell shock was believed to be the result of a physical injury to the nerves. In other words, shell shock was the result of being buried alive or exposed to heavy bombardment. The term itself had been coined, in 1917, by a medical officer called Charles Myers
First, most historians are very reticent to "diagnose" people of the past for two reasons. One, diagnosis and medical understandings are greatly influenced by cultural understandings of "normality" and "health." Without similar definitions, one cannot opine a real diagnosis. Two, these differing understandings mean that different information or "symptoms" are collected than what medical professionals would consider to be relevant for current diseases and mental illnesses. Just look at the recent controversies regarding the updating of the DVSM for a small taste of changing norms and guidelines.
Second, World War I was a crucial period in the formation of modernity and psychiatry. Paul Lerner, Mark S. Micale, and Edgar Jones are prominent historians in this field. I suggest Micale's article in The Psychiatric Times: "The New Historical Trauma Studies."