What is a delta symbol?

A delta symbol is the Greek letter delta, written as uppercase Δ and lowercase δ, and it is used in many fields to represent change, difference, or a specific quantity. The uppercase delta Δ looks like an equilateral triangle and is the most recognized form. In mathematics and science, Δ means change or difference. For example, Δy over Δx means change in y divided by change in x, which is slope. In physics, Δv means change in velocity, ΔT means change in temperature. In chemistry, ΔH means change in enthalpy, ΔS means change in entropy, and δ means partial charge. In finance, Δ or delta means the rate of change of an option price relative to the underlying asset. The lowercase delta δ has additional meanings. In calculus, δ represents a very small change, often paired with epsilon ε in limit definitions. In chemistry, δ+ and δ- represent partial positive and negative charges in polar molecules. In astronomy, delta is used to denote the fourth brightest star in a constellation, like Delta Orionis. The symbol comes from the Greek alphabet, where delta is the fourth letter, derived from the Phoenician dalet. On a keyboard, you can type Δ by using Alt codes or inserting symbols, and in HTML it is written as Δ for uppercase and δ for lowercase. It is one of the most important symbols in STEM.