The Definition of Thriller

There is not a narrow definition for thriller. A thriller is made up by the sudden rush of emotions, the excitement, and sense of suspense, apprehension, and exhilaration that drive the narrative, sometimes subtly with peaks and lulls, sometimes at a constant, breakneck pace." In short, a thriller is exactly what it says it is. It thrills. The following is quoted on www.findmeanauthor.com
“Thriller is a genre of fiction in which tough, resourceful, but essentially ordinary heroes are pitted against villains determined to destroy them, their country, or the stability of the free world. Part of the allure of thrillers comes from not only what their stories are about, but also how they are told. High stakes, non-stop action, plot twists that both surprise and excite, settings that are both vibrant and exotic, and an intense pace that never lets up until the adrenalin packed climax.”
Thriller novels provide a rich literary feast embracing a wide variety of worlds - the law, espionage, action-adventure, medicine, police and crime, romance, history, politics, high-tech, and religion.Thrillers are usually about life and death situations. Other examples of this genre in literature include The Da Vinci Code, The Hunt for Red October, The Day of the Jackal, and Jurassic Park.
The Thriller fiction genre is a genre of literature that typically entails fast-paced plots, numerous action scenes, and limited character development. One example is that: the hero, who may even be an ordinary citizen drawn into danger and intrigue by circumstances beyond their control faces danger alone or in the company of a small band of companions. The protagonist may be a law enforcement agent, a journalist, or a soldier, but typically he or she is cut off from the resources of "their" organization.
Part of the allure of thrillers comes from not only what their stories are about but how they are told. The plot of a thriller is usually driven by the villain, who presents obstacles that the hero must overcome.