A fossil is any preserved remains, impression, or trace of any once-living thing from a past geological age. Fossils are the geologically altered remains of a once-living organism and/or its behavior. There are two main types of fossils:

• Body fossils: These represent all or part of the organisms physical remains, such as bones, shells, teeth, or wood.

• Trace fossils: These are the evidence left by organisms in sediment, such as footprints, burrows, and plant roots.

Fossils can be used to determine the relative age of different rock strata and to study the evolution and adaptation of plants and animals to their environments. Paleontology is the study of fossils, including their age, method of formation, and evolutionary significance. Specimens are usually considered to be fossils if they are over 10,000 years old. The oldest fossils are around 3.48 billion years old to 4.1 billion years old. Fossil collecting is the collection of fossils for scientific study, hobby, or profit.

Fossils occur commonly around the world, although just a small proportion of life makes it into the fossil record. Most living organisms simply decay without trace after death as natural processes recycle their soft tissues and even hard parts such as bone and shell. The term "fossilization" refers to a variety of often complex processes that enable the preservation of organic remains within the geological record.