One who believes in an idea as "fundamental," the absolute, literal truth, adheres to the ORIGINAL version of the idea, is prepared to take action based on that belief, and is unlikely to change their mind.
Often misdefined as "fanatic". A person can be a fanatic and a fundamentalist, or only a fundamentalist, or only a fanatic, or neither.
The action taken by a fundamentalist depends on the nature of their belief. It can be a personal action which affects no one else, or an action which affects tens of thousands of others, or something in between. Can be positive or negative.
Can apply to any religion or ideology, also of a method of or pertaining to fundamentals.
Two of the most common usages:
"islaamic fundamentalist" - generally stereotyped as jihaadist. Not necessarily so.
"christian fundamentalist" - generally stereotyped as bigoted and in-your-face. Again, not necessarily so.
And a correct but uncommon use:
Music teacher: "You need to learn your instrument's fundamentals before you can play solos."
Kid: "Teacher is a fundamentalist."