persistence length

in polymers
https://doi.org/10.1351/goldbook.P04515
The average projection of the end-to-end vector on the tangent to the chain contour at a chain end in the limit of infinite chain length. The persistence length (symbol a) is the basic characteristic of the worm-like chain.
Source:
Purple Book, 1st ed., p. 51 (http://old.iupac.org/publications/books/author/metanomski.html)