Rather than giving a definition of religious fundamentalism, I think I can better define it by listing what the characteristics are of a fundamentalist. Most of these characteristics I think are "all or none", but I suppose a religious fundamentalist may display a subset of these.
Keep in mind, most of these characteristics I could add a phrase at the end or beginning: "Without any objective evidence whatsoever....", so I will refrain from repeating that.
So here I go:
1. A fundamentalist believes (claims) that the single most important aspect of their life IS their religion and the practice of their religion.
2. A fundamentalist believes (claims) that their relationship with an unknown and unseeable God is the single most important "relationship" - trumping relationships with their spouse, children, parents, etc. Thankfully, almost every fundamentalists only makes this claim, and their day-to-day activities and decisions prove that they truly do not believe this. But it is a duty of every good fundamentalist to at least state this claim in some form or fashion.
3. A fundamentalist believes - no, they KNOW - their religion is the ONE religion that gives a pathway to heaven and hell, and those who are outside of their religious sect are headed to an eternity of damnation. And this they hold to with great fervor, without any doubt whatsoever, despite a lack of any objective evidence whatsoever.
4. A fundamentalist will believe their holy text, as written in English, is completely true, without any error whatsoever, and completely whole containing answers to all of life's issues - AND, they believe that anyone who doesn't agree with this tenant is not a true member of their faith community, and is therefore lost and going on to eternal damnation. This was one of the key points of the Southern Baptist conservative resurgence - and many good men were destroyed over this tenant.
5. A corollary to #3: A fundamentalist will actually worry (i.e. spend time contemplating and losing some measure of their happiness) that their loved ones who don't believe the above are going to spend all of eternity burning in an unquenchable hell-fire, and never being consumed.
My fundamentalist friends will unashamedly say "Why, yes, numbers 1 through 5 ARE what I believe; you mean YOU don't???" And thus, there is no way I could be a "real" Christian and not agree with #1 through #5 above.
But, I am not an atheist. I am a Christian. And I maintain that my views on #1 through #5 are none of your business - maybe I do, maybe I don't believe some or all. But I am a Christian. Period. But sadly, to fundamentalists, that is not good enough. To not give assent to all five statements makes me not a true believer, and I'm headed to the lake of fire. Forever and ever, never to be consumed or extinguished. Crap.
The journey out of fundamentalism for me is one that would take many chapters to explain, so I won't do that here. I'll just say I thank God - yes, I thank God - that I am no longer a fundamentalist.
Oh, and what is legalism? I'm not sure, I would have to leave that up to the fundamentalists to determine.