What is the real meaning of delta?

The real meaning of delta is change, difference, or the mouth of a river, depending on whether you are talking about symbol or landform, but both come from the same Greek root. In its most original and universal sense, delta is the fourth letter of the Greek alphabet, uppercase Δ and lowercase δ, which comes from the Phoenician letter dalet meaning door, shaped like a triangle. The ancient Greeks used this triangular letter to describe the triangular landform at the mouth of the Nile River, giving us the geographical meaning. In mathematics and science, which is the most technical real meaning, delta means change or difference, defined as final value minus initial value. This is the real meaning used in calculus, physics, chemistry, and engineering. For example, Δx means change in position, ΔT means change in temperature. In finance, the real meaning is sensitivity, how much an option price moves relative to the stock. In everyday language, the real meaning has expanded to mean any difference, gap, or improvement area, such as delta between plan and actual. So there is not one single real meaning, but the two authentic roots are the Greek letter meaning door and shaped like a triangle, and the scientific meaning of change. Everything else, including company names like Delta Airlines, which was named for the Mississippi Delta region, derives from those two core ideas.