Delta: Fourth letter of the Greek alphabet

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, the upper-case letter Δ can be used as symbol to represent "change" in a variable (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.

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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