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.
The upper-case letter Δ can be used as symbol representing "change".
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.