The belief that a god created the earth and all its creatures in six days, not more than a few thousand years ago.

"Creationism" is unscientific but pretty much all scientists and naturalists were creationists before the 20th century. But at the beginning of the 20th century, the advance of scientific information and methodology caused creationism to be rejected by many scientists because of the moral demands the creationist bible made upon those scientists. These demands included believing the creation of a fully functional earth and living organisms upon it from nothing in just seven days, that there is no evolution of living creatures, and the explanation of geology through catastrophism not evolution (yes, landscapes evolve too).

CentrX has pointed out that creationism is not necessarily specific to the biblical interpretation of the origin of man, however it is one aspect that needed mentioning.