Fundamentalism came to be most closely associated with the belief that the Bible is literally inerrant. The fundamentalists believed the allegorical or metaphorical interpretations of the Bible favored by modernists were all wrong. The Catholic Church and older Protestant denominations had always invested the Bible with great authority, but fundamentalists took rigid literalism to new and extreme degrees.
Since then, the word "fundamentalist" has been used to describe reactionary movements in Islam, Judaism, and other religious traditions with entirely different doctrines. It also has been extended to conservative Christians who don't necessarily agree with the original "fundamentals." What is it that makes a religion "fundamentalist"?
If the topic interests you, I highly recommend Karen Armstrong's book The Battle for God. She analyzes the histories of fundamentalist movements in Christianity, Judaism, and Islam, and calls out what they have in common. What emerges, to me, is that fundamentalist movements primarily are backlashes to social change that find expression in religion.
Fundamentalists are motivated by existential fear more than by devotion. They see themselves in danger of annihilation from something they think the modern world is forcing on them.
Traits common to all fundamentalist movements include --
They embrace dualistic, black-and-white, absolutist thinking. Everything and everyone is either good or evil.
They fear annihilation.
They take their values from an idealized past, either real or imagined.
They often withdraw from mainstream society to form their own communities and cultures.
They see themselves in a struggle to retake society and reinstate whatever they think the modern world is destroying.
The point is that people can be deeply and traditionally devout without being fundamentalist. Belief in God or Allah or the Pure Land is not in itself fundamentalist. Investing scripture with sacred authority is not necessarily fundamentalist. A traditionally religious person who is open to new understanding and tolerant of other views is plainly not a fundamentalist.