Delta in Modern Use

Delta's best-known modern use has classical roots of its own, distinct from the alphabet's numerical and scriptural history covered elsewhere in this atlas: the historian Herodotus, writing in the 5th century BCE, described the triangular landform where the Nile splits into channels before reaching the sea as shaped like the letter delta, and geographers have called any such landform a river delta ever since, making delta one of the few Greek letters to lend its name to a geological feature well before the modern scientific era.

In mathematics and the sciences more broadly, an uppercase delta denotes a change or difference between two values, as in a temperature difference or a change in a variable across an equation, a convention formalised in the 18th and 19th centuries alongside the rise of calculus; the related epsilon-delta definition of a mathematical limit, developed by Augustin-Louis Cauchy and Karl Weierstrass in the 19th century, pairs delta with epsilon in one of the most widely taught proofs in calculus. During the covid-19 pandemic, the World Health Organization's 2021 system for naming virus variants after Greek letters made Delta the public name for the B.1.617.2 lineage, a use with no connection to any of the letter's older meanings.