δ | delta

The Greek letter δ (delta) was derived from the ancient Phoenician letter 𐤃 (dālet) which, in turn, was derived from 𓉿, an Egyptian hieroglyph depicting a door.

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

  dU = δQ – δW

  where U is internal energy, Q is heat and W is work.

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