Creationism is the account for the existence of the universe and of human life that says that both were created ex nihilo by a Supreme Being. The argument is that everything that exists now must have a preceding cause, that the chain of causation cannot be infinite, and therefore that there must be a first cause, which is God. The argument is not persuasive to those who ask what it was that caused the Supreme Being.
Creationism falls into the category of "plausible stories." There appears to be nothing logically inconsistent with the view that some superior race (or even eternal being) seeded the universe or perhaps only seeded the Earth with human being. There likewise appears to be nothing logically inconsistent with the view that some superior being created the whole universe. Physicists who study the history of the Universe almost universally agree with the idea that the universe, and all of space and time, originated with a singular event called the "Big Bang." Physicists are currently still pushing their understanding of the events back toward "time zero." Furthermore, they are trying to understand why the universe should have one set of characteristics and not another. But they do not simply posit a supreme being who decided these things on the basis of personal preference.
The nature of science is that it tries to find the reasons for things, and to do so every conclusion must be grounded in evidence and woven together in an internally consistent logical account. While nothing may be absolutely certain, since the next observation may turn out to stomp on the physicists' metaphysical sand castle, everything is grounded in ever growing accumulations of solid evidence, empirical observations.
A general definition of creationism does not preclude a creation that occurred millions or billions of years ago. But in practice, the term usually refers to a specific version of creationism, sometimes called "young earth creationism". This version asserts that there was a creation about 6,000 to 10,000 years ago. Such a date is consistent with a literal interpretation of the Hebrew scriptures, and is accepted by fundamentalist Judaism, Christianity, and Islam. The 17th century Anglican Archbishop James Ussher, by these scriptures, calculated the precise date of creation to be October 23, 4004 B.C. Their ideas are grounded in holy texts, not in the evidence provided to the senses of a community of observers. If one does not accept what they term "holy writ," then there is nothing holding their accounts up other than that they appear to be plausible.
Some cultures are typified by the belief in a creator God, some cultures believe that the Universe has always existed, and some cultures go no further than asserting that God is a superior being who is concerned about the welfare of human beings. Creationism is characteristic of only the first belief system.