In Wikipedia, it says that "The evidence for QCD condensates comes from two eras, the pre-QCD era 1950–1973 and the post-QCD era, after 1974. The pre-QCD results established that the strong interactions vacuum contains a quark chiral condensate, while the post-QCD results established that the vacuum also contains a gluon condensate."
I believe the quark condensate is a pretty standard story we learn from, for example, any text on QCD (see Dynamics of the Standard Model by Donoghue et all). How about the glon condensate? What is the proper expression and analytic computation of gluon condensate?