# D (disambiguation)

D is the fourth letter of the Latin alphabet. It can also refer to the following:

## Computing

• D, the name of various programming languages:
• D, the "Minimal Protection" security division in the TCSEC
• d (short for daemon), a suffix often added to the name of a computer program that runs as a background process
• .d, a common pathname component suffix for a directory for a program or subsystem on POSIX and POSIX-like systems; for example "/etc/init.d" would be a directory which contains files associated with the init program.
• .d, a file format used for Agilent MassHunter mass spectrometry software

## Music

### Bands and musicians

• D (band), a Japanese rock/metal band
• "The D", one of the names of comedy rock band Tenacious D
• "d:" or "d:?", often used to refer to the Christian rock band Delirious?
• "D!" or "Dee!", names of Detlef Soost, a German dancer and choreographer
• D, the bass player for Australian band Testeagles

## Science

### Chemistry and biology

• D, the symbol for aspartic acid in biochemistry
• D, the symbol of deuterium, an isotope of hydrogen, in chemistry
• Vitamin D
• ATC code D Dermatologicals, a section of the Anatomical Therapeutic Chemical Classification System

### Mathematics

• d, the symbol for the total differential operator (partial differentials use ∂)
• D, often used as a digit meaning thirteen in hexadecimal and other positional numeral systems with a radix of 14 or greater
• Blackboard bold $\mathbb{D}$, representing the unit disk in the complex plane, or the decimal fractions (see number)
• d, often a variable for the diameter of a circle in geometry
• D and d, the derivative operators
• D, the number five hundred in Roman numerals

### Physics

• $\mathbf{D}$, the recommended symbol for electric displacement field
• d, the recommended symbol for: thickness, diameter, relative density (mechanics), lattice plane spacing (solid state physics), and degeneracy of vibrational mode, in molecular spectroscopy
• d, the symbol of the deuteron in particle physics
• D, the symbol for debye, a unit of electrical dipole moment
• D battery, a standard size dry cell battery in electronics
• D, the name of mesons containing charm quarks in particle physics
• D, the recommended symbol for diffusion coefficient in molecular physics
• In solid state physics:
• D, the standard abbreviation for an electronic diode, used e.g. in circuit diagrams

### Other science

• D region, part of the ionosphere in earth science
• D, standing for demand in economics
• d (deci-), the SI prefix meaning one tenth, 10−1
• d, the symbol for day in metrology
• d', psychophysical discriminability