A quick google search reveals that this term was coined relatively recently, (within the last hundred years), and that society is still in the process of hammering out what will ultimately be the firm definition for it.
My observations have led me to conclude that a fundamentalist is best described as a person who believes that he is clinging to ancient truth, divinely revealed, that he feels mankind has largely fallen away from. The fundamentalist mentality is very much left-brained, literal, legalistic, and rigid.
Does that make it a strictly religious term, or a strictly derogatory term? I don't think so. In some situations, a no-compromise, let's get back to basics, attitude is appropriate. The American form of government is a prime example of something which should be approached from a fundamentalist perspective. We began with a set of declared principles that were brilliant, inspired even, and through legislation and judicial rulings, we have been screwing them up, getting further and further away from original intent, ever since.
When it comes to religion, fundamentalism was a place where I once found comfort, but ultimately had to concede that a "package deal", all or nothing, mindset; a belief that we originally started out with something that was perfect, completely error free, does not stand up to scrutiny. No reasonable person could believe that the prophet-warrior Joshua literally made the earth stop turning for a day. Having watched the way religious fundamentalists will rationalize away anything they find in scripture or secular history, which stands in contradiction to their pre-conceived notions, as well as they way they have treated me differently after learning that my faction of Christianity is a different one than theirs; while I won't join the voices that blanketly condemn and spout inflammatory rhetoric likening the "Religious Right" to the Taliban, I do conclude that it does foster some attitudes that are not only flawed in their reasoning, but are unhealthy to the person believing them as well.
As someone with a Mormon background, this topic inevitably leads me to the question, is Mormonism a fundamentalist religion?
Mormons believes that their faith represents a restoration of lost truth. There absolutely are fundamentalist aspects to it...but they also believe that communication with God is ongoing, and that this is a special time in history, with truths being revealed in modern times that have heretofore not been given to mankind. To a fundamentalist, his ultimate source of truth is a book of ancient scripture. To a Mormon, his ultimate source of truth is a prophet who lives today, who has the full authority to clarify and expand upon the teachings of any prophet which came before him.
From my studies, I am aware of five unique new teachings introduced by Brigham Young: Adam-God, blood atonement, Jesus being begotten by sexual intercourse between God and Mary, non-polygamists being relegated to a subservient position in heaven, and that the Blacks would not receive the priesthood until after all the other decedents of Adam had the opportunity. None of these are accepted as the authoritative position of the Church of Jesus Christ of Latter-day Saints today.
If Mormons were truly fundamentalists, they would not feel at liberty to dismiss troubling parts of the discourses of the early brethren as unauthoritative speculation. That is why those who have a fundamentalist mindset when they convert will eventually will eventually make a choice, they will either accept some compromises on the rigidity of their belief system, or they will leave the church to associate with polygamists.