What are side chain order parameters?

I was reading an [article](http://dx.doi.org/10.1016/j.jmb.2008.05.006):



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> We first compared experimental (S 2 exp) and computed (S 2 calc) side-chain order parameters using the simplest approximation of side-chain conformational variability, where motions are only allowed within the rotamer well of the side-chain conformation observed in the crystal structure 
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Now I am wondering what side-chain order parameters are.


