An index fossil is a fossil that is used to define and identify geological periods or faunal stages. These fossils are typically widespread, abundant, and limited to a short range of geological time. Index fossils are crucial in biostratigraphy, helping to correlate rock layers across large distances and establish the relative ages of strata.
Reference: Benton, M. J., & Harper, D. A. T. (2009). Introduction to Paleobiology and the Fossil Record. Wiley-Blackwell.