The short and sweet answer to that question is “A fossil is the remains or evidence of any creature or plant that lived on the earth in a past geologic age.”

But there are so many KINDS of fossils. A more important question for a curious student is “What kinds of fossils are there?” The answer to that question will take a bit more exploration. You'll have to dig a little deeper…pun intended!

The Long Answer
There are several fossil classification systems in use today, but the one that I like the best is the one used by Peter Larson and Kristin Donnan in their book, Bones Rock! They group fossils into two categories: Type I-the remains of the dead animal or plant or the imprint left from the remains.

Type I includes:

bones
teeth
skin impressions
hair
the hardened shell of an ancient invertebrate (an animal without a backbone) like a trilobite or an ammonite
impression of an animal or plant, even if the actual parts are missing.

So now you have one short and one long answer to the question: "What is a Fossil?" Let's build on that.

Type II- Something that was made by the animal while it was living that has hardened into stone. These are called trace fossils.

Type II includes:

footprints
burrows
coprolite or animal poop

Type I fossils can be the actual thing that it once was, like a piece of bone or hair or feather. More often the bone material is replaced by different minerals contained in the liquid of the sediments that buried it. What was once bone is now some sort of crystal or mineral.

This process also takes place with shells, exoskeletons and wood. If the spaces in the bone are filled with liquid minerals which later harden it is called permineralization.

Sometimes the organic material is dissolved by the mineral-laden water. The process happens so slowly that each cell is dissolved and replaced by a particular liquid mineral before it hardens. This is called petrification. In petrification, every detail down to the cellular level is duplicated in the minerals.

Type I can also be molds or casts of the original animal or plant part. If the original organism decays, leaving an imprint and an empty space, it is called an exterior mold or simply a mold. If a space in the structure is filled with minerals as the original animal or plant part dissolves, it is called a cast.

So now you have the short answer and the long answer to the question "What is a fossil?”

Was that more info than you were seeking? I hope not! Fossils are the illustrations on the pages of rock that are the earth's history. I think the more you know, the more you'll want to discover about these fascinating traces of life we call fossils.