A hypothetical linear chain molecule, free from short-range and long-range interactions, consisting of infinitely thin rectilinear segments (bonds) of fixed length, jointed at fixed bond angles; the torsion angles of the bonds can assume all values with equal probability.
Source:
Purple Book, 1st ed., p. 50 (http://old.iupac.org/publications/books/author/metanomski.html)