A polymer showing crystallinity. One or two-dimensional order leads to mesophase structure. The range of order may be as small as about 2 nm in one (or more) crystallographic direction(s) and is usually below 50 nm in at least one direction.
Source:
Purple Book, 1st ed., p. 74 (http://old.iupac.org/publications/books/author/metanomski.html)