What is meant by cognitive dissonace?

A good example is smoking. Most people know that smoking is bad for their health, however, they still smoke. This separation between beliefs and behaviour is cognitive dissonance. It is often described as mental discomfort. This theory was originally established by Leon Fessinger, who also established that the most common way that people tend to fix their cognitive dissonance, is to change the way they perceive their actions, such as finding excuses. Going back to the previous example, that could be stress. People know smoking is bad but they have to smoke because they feel stressed.