What the delta symbol means
Delta (Δ, small δ) is the fourth letter of the Greek alphabet, and its capital is one of the most used symbols in science. Δ means "change in": Δx is the change in x, ΔT a change in temperature and Δt a span of time. Small δ usually means a tiny change, or a partial charge in chemistry (δ+ and δ−).
In algebra, Δ is the discriminant of a quadratic, b² − 4ac. In chemistry, ΔH is the enthalpy change of a reaction, and a Δ written over the reaction arrow means heat is applied. In finance, an option's delta tells you how much its price moves when the underlying asset moves by one unit.
A river delta is named after the letter, because the land at the mouth of the Nile is shaped like a Δ. In bios and logos, Δ is often used simply as a triangle; for more shapes see the triangle symbol page.
Δ, ∆ and ∇: which one should you use?
Three characters look like a triangle. Δ (U+0394) is the Greek capital letter. ∆ (U+2206) is the math "increment" sign, which is what a Mac types with Option + J. They look almost the same, but software treats them as different characters, so pick one and stick with it.
For Greek text, physics and chemistry, Δ is the safe choice. ∇ is nabla, an upside-down delta used for the gradient in vector calculus, and △ is a geometric triangle, as in △ABC. Small δ has a look-alike too: ∂ is the partial derivative sign, a curly d rather than a delta.
How to type delta
On a Mac, Option + J types ∆. On Windows, Alt + 235 on the number pad types small δ. The capital Δ has no Alt code, so in Word type 0394 and press Alt + X, or insert an equation and type \Delta followed by a space. In Google Docs, use Insert > Special characters and search "delta".
On Android, Gboard has ∆ on its second symbols page: tap ?123, then =\<. On iPhone, add the Greek keyboard (the D key types δ) or copy Δ from this page.
In LaTeX, use \Delta, \delta and \nabla; in HTML, Δ and δ. Sums and other Greek operators are on the sigma symbol page, and the rest of the alphabet is under Greek letters.