The fourth letter our alphabet (“D” if you were wondering) traces its roots to the fourth letter Daleth (top left image) from the Phoenician alphabet, represented by a three sided triangle and translated into “door”. Depicted as ℸ (image second from left) in Hebrew, as 𝚫 (image third from left) in Greek, and “4” in our numerical system. Usage of the word “delta” includes the description of alluvial deposits at the mouth of a river (image fourth from left) where the river meets a larger body of water (which can take on the triangular appearance of the Greek letter). In mathematics it can be understood to mean the difference between two values. Its meaning and use in regards to fingerprints (and latent prints) is based on its visual appearance similar to that of the delta of a river. The text, The Science of Fingerprints defines it as “that point on a ridge at or in front of and nearest the center of the divergence of the type lines” (images fifth and sixth from left). The delta of a fingerprint is a key focal point useful in attributing a pattern type (loop, whorl, arch). For fingerprint classification it is a locatable point in space upon a ridge but the more general appearance of the delta area is three sided (like the Greek letter) and is simply the convergence of three systems of ridges which during fetal development formed in separate locations and eventually spread across the finger to the same point. This type of ridge convergence also occurs on the palmar surfaces of the hands and the plantar surfaces of the feet which also are characterized by the friction ridge skin.
Federal Bureau of Investigation, The Science of Fingerprints: Classification and Uses, U.S. Department of Justice, U.S. Government Printing Office: Washington DC, 2006.