Fundamentalism is belief in the Fundaments of a religion. Let me show you what dictionary.com came up with.
fun•da•ment P Pronunciation Key (f n d -m nt)
n.
1.
a. The buttocks.
b. The anus.
2. The natural features of a land surface unaltered by humans.
3. A foundation, as of a building.
4. An underlying theoretical basis or principle: “All neighbor states... must revise... their policy fundaments” (C.L. Sulzberger).
So what are the buttocks, or basis, of any given religion? This would obviously depend upon the religion in question. For Christians, regardless of their denomination, this should mean belief that Jesus is the Christ, or Messiah. Otherwise you wouldn’t call yourself a Christ-ian. What you believe this term Messiah means, along with your view of how the bible should be interpreted, must be secondary.
For Muslims, the Shahadah, statement of faith, must be the underlying principle. This goes, (in English,) "I testify that there is no god but God, and Muhammad is the messenger of God."
And this is where it should stop. That is the only thing that can be called the fundament of Islam, as that is what makes you a Muslim. Yet this is where the dogma starts to creep in, right from the beginning. The Shahadah is often mistranslated as “Muhammad is the last messenger of God”. It may be a well meaning change, but it is no longer the Shahadah, and someone who reads it out does not become a Muslim, in the strictest Fundamentalist sense.
This is why I believe those who call themselves fundamentalists in the world do not really believe in the fundament of their religion. They believe in their own separate fundaments of their own individual religions. They hold the outer material aspects to be more important than genuine belief, and intend to enforce these made-up peripheral doctrines as if they were the fundament of the religion. They prefer to tell other people what to do and run other people's lives than to correct and improve their own lives.
So I advise all of you to be wary of people who seek to set up a Fundamentalist State. They aren’t intending to enforce the basis of your own religion, be it Christianity or Islam. They simply wish to gain power, and use religion as an excuse to dominate other people from their ego.
I would like to confirm that when I say "fundamentalist", I am not referring to religious conservatives. I do not find textual literalism a particularly appealing basis for religious belief, but the only thing I have a real problem with is when evangelising takes a form intended to be socially divisive, which I have experienced amongst the Muslim community in Britain. In particular, the manner in which various groups try and create hierarchies of understanding where they do not belong. The nastiest groups love to call themselves “fundamentalist”; it’s a much more pleasant term than “ultra conservative extremists”.
My last paragraph only really refers to Muslim extremists, since I don’t know any Christian groups who are trying to become the government rather than simply influence it. (This isn't the place to rant about "Christian Democrat" political parties, which I understand are generally not fundamentalist)