The lower-case letter δ has many uses in maths and sciences, such as:
- in astronomy: the fourth brightest star in a constellation. For instance, Mintaka is designated as δ Orionis.
- in chemistry: δ bond, a type of covalent chemical bond formed by overlapping lobes of d orbitals
- in chemistry: a symbol for partial charge, δ− or δ+
- in earth sciences: to express stable isotope ratios, for example δ2H, δ13C, δ18O
- in mathematics: δ, the first Feigenbaum constant (approximately 4.669201)
- in maths and science: Dirac δ function
- in neurophysiology: δ-waves, a type of brain waves detected by electroencephalography
- in pharmacology: a type of opioid receptor
- in thermodynamics: inexact differential. For example, the first law of thermodynamics is expressed as
where U is internal energy, Q is heat and W is work.
dU = δQ – δW