So far I have defined atheist and Christian. Today, I'd like to tackle another word that gets used by many but whose definition is elusive: fundamentalist.
First, I'd like to explore how the word seems to be used by people. I'll then get to how I try to use it, and why.
There are three meanings that I have seen the word "fundamentalist" used for.
First, there is the historical origin of the term, to refer to those people who accept the doctrines outlined in the series of essays titled The Fundamentals: A testimony to the truth, published between 1910 and 1915 [1,2,3,4]. This definition would mean that only those Christians who accept these doctrines (creationism, virgin birth of Jesus, the atonement, and others) are true "fundamentalists" - only they hold to those particular fundamentals [3,5,6].
Second, there is the obvious extension to other dogmatic positions. Perhaps anyone who dogmatically accepts a particular set of doctrines as true is a fundamentalist [3,6,7]. This could include some (but not all) members of most major world religions. I think some religions have more of a tendency to this sort of fundamentalism than others. It is also not unreasonable to apply this definition to other beliefs - for example atheism (though I don't think you'll find many fundamentalist atheists by this definition) or political ideologies [6,7].
Third, I feel that people are increasingly using the term fundamentalist as a slur - to mean little more than "somebody who passionately believes something that I disagree with" [4,7]. I've seen this meaning used by humanists (including myself) to refer quite broadly to a range of conservative Christians; I've also seen the term used in this sense by Christians to describe a wide range of atheist writers.
So those are three definitions that are used for the term fundamentalist. I suspect that they represent points on a continuum of meanings, and that some mix of these three definitions is often in people's minds when using the term. But let's consider these three definitions in particular.
The first definition, while historically well-motivated, is so narrow that it's not very useful for general discussion. Very few discussions need to refer specifically and exclusively to the original Fundamentalists, and these could be distinguished by capitalization (as I've done in this sentence) or by explicitly referring to The Fundamentals as their statement of belief.
The third definition is neither historically well-motivated nor particularly informative: we have plenty of words to use when we find someone's position distasteful, and adding one more is unlikely to help us communicate any better. (Yes, I am assuming that the purpose of language is to help people communicate. Call me an optimist.)
So, as the more astute of you may already have guessed, I'm opting for the second definition:
A fundamentalist is someone who dogmatically holds to a set of beliefs as true. (As opposed to tentatively holding to beliefs and being willing to revise those beliefs in the face of opposing evidence.)This definition covers a wide enough range of beliefs to be relevant in general conversation, while remaining specific enough to be informative. For example, I know some Christians who are fundamentalists under this definition, and others who are not. I don't know any atheist whose position could be called fundamentalist in this way, but I'm fairly sure that some must exist. I know some very woo-oriented people whose positions are fundamentalist (the conspiracy-theory approach to anti-vax, for example), but I've also known people who seem to be honestly willing to follow the evidence. (These latter are generally now non-woo, simply because the evidence always points in another direction.)
Can you think of people with fundamentalist attitudes in other areas of life? With non-fundamentalist attitudes who might be branded fundamentalist? Is there a belief, community, or identity that you think is inherently fundamentalist? Inherently non-fundamentalist? Let us know in the comments.
Now, I've said it before, and I'll keep on saying it: I'm not trying to impose a definition on others. I am a linguist (and so an expert of sorts), but language (like science, and like truth itself) does not get handed down from authorities. Nor are the meanings of words decided by some noble democratic process. Meaning in language emerges by a sort of quasi-Darwinian selection, in which people participate only semi-consciously - a sort of mob-consciousness. Meanings that fit the speaker's and the listener's purposes are propagated; meanings that do not fit are not propagated.
 Online text of The Fundamentals
 Wikipedia entry on The Fundamentals
 Wiktionary definition of fundamentalist
 Ontario Consultants on Religious Tolerance essay on the term fundamentalism
 Dictionary.com definition of fundamentalist
 Cambridge Advanced Learners Dictionary definition of fundamentalist
 Wikipedia entry on fundamentalism
 Oxford English Dictionary definition of fundamentalist (access not free)