It is almost always used by a group of people that want to reach back into time and use something that they're willing to idealize and then they want to follow that version of history or religion or whatever it is without acknowledging any flaws that existed at that time. You could also look at some people who want to interpret the constitution strictly and hold everything up to it as though it wasn't a changing document at the time of its conception.
As the previous posts suggest, there is the connotation of extremism in Fundamentalism. Miss Maudie of Harper Lee's To Kill a Mockingbird elaborates upon the religious fundamentalists as she tells Jem and Scout about the Radley household,
"...sometimes the bible in the hand of one man is worse than a whiskey bottle in the hand of--oh, of your father."
In another literary allusion, Nathaniel Hawthorne offers a critical portrayal of the fundamentalists, the Puritans.
I like the definition given in #3 which seems to capture the essence of fundamentalism. It is a term that designates a group of people that think they are completely right and that therefore everyone else is wrong. It does not allow for any doubt or wavering. Of course the term is applied mostly to relgious groups such as fundametal Christians and fundamentalist Islam.
The term originated during the 1920's from a pamphlet entitled "The Fundamentals," which advocated a literal interpretation of the King James Version of the Christian Bible. It's origin was largely a result of the "back to the basics" thinking that influenced Americans after the tragedy of World War I. It's origin in time is close to that of the rejuvenated Ku Klux Klan and the Prohibition movement which led to the 18th Amendment. It's most famous proponent was William Jennings Bryan, who took it upon himself to prosecute John Scopes for teaching Darwinism, which Bryan and most fundamentalists construed to mean humans were descended from apes, and Darwin's theories as being in conflict with the Creation story of Genesis. Interestingly, the debate still rages on, although few realize its origins.
Fundamentalism was first used to describe the literal interpretation of the Bible as fundamental to Christian teaching and also the adherence to such beliefs. It has evolved into the political world to describe someone who adheres to a strict set of principles or beliefs.
One of my favorite professors once described fundamentalism for me, and I think he may have noted that he was paraphrasing an ex-president; perhaps Jimmy Carter. He said that whenever you’ve become certain that you can’t be wrong, you have become a fundamentalist. This was in a much larger conversation about how total objectivity is impossible, every system of thought is flawed and that no one person, religion or political ideology is infallible. But in current political contexts, a fundamentalist is someone who is certain that they are right – about a favored political system, religion and so on, and some fundamentalist groups are willing to resort to violence to protect their way of life/beliefs. Governments, individuals and businesses can be considered ‘fundamentalist’ – the term is often used to refer to religious zealots and terrorist groups but it is not limited to them. But it can also be applicable on an individual basis or in some other discipline. If you have made up your mind – about anything at all – and you are absolutely certain you are right, then you are a fundamentalist. It doesn’t matter what it is. Scientists are fallible but they pride themselves on remaining objective and always questioning motives, events, causes and effects. Because the history and evolution of scientific development has often resulted in the overturning of older established theories about the world, many scientists understand the necessity of continuing a healthy skepticism of their own beliefs. Hence, a flat-world fundamentalist would kick himself in hindsight.
This word is most typically used to refer to people who believe in the absolute truth of the scripture or beliefs of their religion. They reject other beliefs as untrue and they do not think critically about their own beliefs. Those who are opposed to fundamentalists will often say that fundamentalists have "blind faith."
Fundamentalism and fundamentalists are generally portrayed in a negative light. People worry, for example, about Islamic fundamentalists and their desire to wage holy war against non-Muslims (see islamic-fundamentalism link below). In the United States, liberals tend to be concerned about the power of Christian fundamentalists who promote things like prayer in school and who do not believe in evolution.
Overall, then, the word "fundamentalism" refers to an uncritical belief that one's own scriptures and beliefs are completely and totally right and that those of others are wrong and flawed.