The Greek letter ξ (xi) was derived from the ancient Phoenician letter 𐤎 (sāmek), which, in its turn, could have evolved from the Egyptian hieroglyph djed, representing the spinal column. Other theories say 𐤎 meant “fish” (actually showing a fish skeleton?) or “a tent peg”.
The lower-case ξ has a number of uses in maths and sciences, including:
- in astronomy: the correlation function ξ(r)
- in natural product chemistry: to denote unknown configuration (which could be either α or β) at a chiral centre
- in mathematics: the remainder term of Taylor’s theorem
- in physical chemistry: the extent of reaction
- in physics: the spatial frequency (the reciprocal of the wavelength λ)