equivalent chain

in polymers
https://doi.org/10.1351/goldbook.E02190
A hypothetical freely jointed chain with the same mean-square end-to-end distance and contour length as an actual polymer chain in a theta state.
Source:
Purple Book, 1st ed., p. 50 (http://old.iupac.org/publications/books/author/metanomski.html)