You may often hear some Christians referred to as a “fundamentalist” or a “fundie.” This is often used in television political analysis (where you will likely hear things such as right wing, left wing, etc.).
What Does the Word Fundamentalist, Fundamental, or “Fundie” Mean?
A fundamentalist (also called a Fundie) is a term that means your are very strict in your adherence or analysis of a given thing. Anyone can be fundamental or a “fundie” about something. It doesn’t always relate to religion, although it is used very often within a religious context.
For example, a Fundamentalist Christian is a person that takes the Bible texts very literally, and is very strict in their analysis and interpretation of scripture, and believes in its inerrancy and truthfulness. They are usually extrememly conservative because of this. This means when it says to avoid something, they interpret that to mean you absolutely need to avoid it.
Anyone can be a “Fundie” on any religion (or any topic for that matter). There are fundamental atheists, Mormons, Catholics, Jews, etc. Anyone can have a fundamental interpretation of something, which simply means they adhere very stricly to what it says and regard it as a fact (or commandment from God Almighty).
Any example of a fundamental atheist is something that strictly adheres to science only, and believes very strictly all popular scientific theories, laws, and so forth. A fundamental atheist would usually be considered a person that totally rejects an idea of a God or higher power, and instead fully believes in nature and scientific theories as the answer of why we exist.