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*)

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