The recovery of fingerprints from a crime scene is an important method of forensic science.
Fingerprints are easily deposited on suitable surfaces (such as glass or metal or polished stone) by the natural secretions of sweat from the eccrine glands that are present in epidermal ridges.
These are sometimes referred to as "Chanced Impressions".
In a wider use of the term, fingerprints are the traces of an impression from the friction ridges of any part of a human or other primate hand.
The most popular systems used the pattern class of each finger to form a key (a number) to assist lookup in a filing system.