Remains or representations of organisms that have died many thousands of years ago or longer.
For example, remains or representations that are approximately 10,000 or more years old can be described as fossils, though obviously this distinction can be a bit arbitrary given old but not quite that old remains.
Fossils come in a variety of types as well as degrees of preservation. They can consist of the original material, they can be mineralized representations of that material, or they can be impressions that the original material made on what has subsequently become surrounding rock (casts, molds, and what are described as endocasts). They can be well preserved, including representations not just of hard tissues (bones, shells) but also soft tissues. Key is burial prior to decomposition of the more easily lost components of organisms.
Different materials that came to surround the organisms following its death and deposition can also give rise to differences in fossil preservation, with fine muds for example providing more information than courser encasing substances. The physical state that a fossil is discovered in is also important, since fossils tend to deteriorate rapidly following their reexposure to the atmosphere and weather.
For additional discussion on fossils, see fossil record. See also fossilization and paleontology.