Delta (uppercase/lowercase Δ δ), is a letter of the Greek alphabet, used to represent the "d" sound in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 4. Letters that came from it include the Roman D and Cyrillic Д.

In mathematics, physics and chemistry, the upper-case letter Δ is used to denote "changes" in a variables (e.g., 👁 {\displaystyle \Delta x}
). It can also be used to represent discriminant—a key algebraic expression used to determine the number of roots a polynomial has. The lower-case letter 👁 {\displaystyle \delta }
, on the other hand, is often used to denote small quantities in proofs involving limit, a key concept on calculus is based on.^[1]

In molecular chemistry, the lower case Greek letter δ is used to represent a partial charge. This is always less than the unit charge associated with an ion, but has no real fixed value.

A river delta is so named because its shape approximates the upper-case letter delta.

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