There's a lot of confusion over the term and a lot of straw men around, so I'm going to try to give as clear and objective an definition as possible.
Fundamentalism as a movement has been around since about the 1870s, but its doctrines and practices have been around since the birth of Christianity.
The term "Fundamentalist" was coined in 1920 by Curtis Lee Laws, in an editorial about a group of conservatives in the Northern Baptist convention.
After dismissing several other terms, he wrote,
"We suggest that those who still cling to the great fundamentals and who mean to do battle royal for the fundamentals shall be called "Fundamentalists."
The terms I've bolded make a good outline for a discussion of Fundamentalism.
Another definition provided by Mark Sidwell in The Dividing Line is:
Fundamentalism is the belief (1) that there are certain truths so essential to Christianity that they cannot be denied without destroying Christianity and (2) that these essentials are the basis of Christian fellowship.
The foundation of Fundamentalism is the Bible as the inspired, inerrant, and authoritative Word of God.
The practice of Fundamentalism is based on the doctrine of holiness - separation from sin and the world and to God.
"The Great Fundamentals"
The most common list of the "fundamentals of the faith" is the "Five Fundamentals" first stated by the General Assembly of the Northern Presbyterian Church in 1910.
1. The inerrancy of the original manuscripts of Scripture
2. Christ's virgin birth
3. His vicarious atonement
4. His bodily resurrection
5. The reality of miracles as recorded in Scripture
(Sometimes #5 is combined with #2 or 4 and "The personal return of Christ to earth" is listed as #5.)
Probably the earliest statement is found in the Niagara Bible Conference's Confession of Faith in 1878.
1. The verbal, plenary inspiration of the Scriptures in the original manuscripts.
2. The Trinity.
3. The creation of man, the fall into sin, and total depravity.
4. The universal transmission of spiritual death from Adam.
5. The necessity of the new birth.
6. Redemption by the blood of Christ.
7. Salvation by faith alone in Jesus Christ.
8. The assurance of salvation.
9. The centrality of Jesus Christ in the Scriptures.
10. The constitution of the true church by genuine believers.
11. The personality of the Holy Spirit.
12. The believer’s call to a holy life.
13. The immediate passing of the souls of believers to be with Christ at death.
14. The premillennial second coming of Christ.
"Battle Royal"
Militancy against false teaching and apostasy has been a hallmark of Fundamentalism since its inception. Militancy is related to Separation as an application of the doctrine of holiness. It is the obedience to the command in Jude 3 to "earnestly contend for the faith which was once delivered unto the saints."
I will probably devote a post to these twin practices when I have time.
Premillennialism
This is the belief that the personal return of Christ to earth will take place before the thousand-year reign of Christ on earth known as the Millennium. Fundamentalism has been predominantly but not exclusively premillennialist.
Dispensationalism
This is a system of organizing history into dispensations according to the way God has dealt with people. It is characterized by as literal an interpretation of prophecy as possible, drawing a distinction between OT Israel and the NT Church. Again, Fundamentalism has been predominantly but not exclusively dispensationalist.
Fred Moritz, in his book Contending for the Faith, has listed these as major distinctives of Fundamentalism.
1. Fundamentalists stand for the Bible as the supernaturally revealed, inspired Word of God.
2. Fundamentalists embrace a doctrinal frame of reference, most commonly identified by "five fundamentals."
3. Fundamentalists show a militant opposition to apostasy, otherwise known as modernism or liberalism.
4. Separatism is a major distinctive of Fundamentalism.
5. Fundamentalism has been, from the beginning, an interdenominational movement.
6. A premillennial viewpoint is prominent in Fundamentalism, though this is not a test of fellowship.
I'll have to stop with this for now but I want to end with a definition of the "ideal" Fundamentalist as stated by David Beale in In Pursuit of Purity: American Fundamentalism Since 1850:
Hannah JoyIdeally, a Christian Fundamentalist is one who desires to reach out in love and compassion to people, believes and defends the whole Bible as the absolute, inerrant, and authoritative Word of God, and stands committed to the doctrine and practice of holiness. . . . Far antedating any present-day organization, however, Fundamentalism is not a philosophy of Christianity, nor is it essentially an interpretation of the Scriptures. It is not even a mere literal exposition of the Bible. The essence of Fundamentalism goes much deeper than that - it is the unqualified acceptance of and obedience to the Scriptures (italics his).