θ state

in polymers
https://doi.org/10.1351/goldbook.Q06240
The state of a polymer solution for which the second virial coefficient is zero.
Notes:
  1. In some respects, a polymer solution in the θ state resembles an ideal solution and the θ state may be referred to as a pseudo-ideal state. However, a solution in the θ state must not be identified with an ideal solution.
  2. The solvent involved is often referred to as 'θ solvent'.
  3. It is assumed that the molar mass of the polymer is high.
Source:
Purple Book, 1st ed., p. 57 (http://old.iupac.org/publications/books/author/metanomski.html)