Delta (upper case Δ, lower case δ) is the fourth letter of the Greek alphabet. In the system of Greek numerals it has a value of 4.

A river delta is named after the letter delta because it has roughly the triangular shape of the upper-case delta.

The upper-case letter Δ is used as the symbol for:

• A macroscopic change in the value of a variable in mathematics or science.
• Any of the delta particles in particle physics.

Deltas are the second lowest caste in Aldous Huxley's dystopic satire Brave New World.

The lower-case letter δ is used as the symbol for:

• A microscopic change in the value of a variable in mathematics or science.
• The Kronecker delta in mathematics.
• The Dirac delta function in mathematics.
• A mark for deletion in proofreading. The usage is said to date back to classical times.

Letters that arose from Delta include the Roman D and the Cyrillic letter De.