I think the term gluon condensate" usually means $<F_{\mu\nu} F^{\mu\nu}/4>$. This is not really a condensate however, because it does not break any symmetry. Furthermore, since the operator $F_{\mu\nu} F^{\mu\nu}$ mixes with the identity operator under renormalization, it is a non-universal quantity, so it is not clear what it means physically.

There are various QCD "sum rules" that make use of this quantity, but they depend on a separation of "perturbative" from "non-perturbative" contributions. I've never understood how this can be done rigorously, and I am not alone in this skepticism, however the practitioners of this art claim that they know what they are doing, and I am not expert to argue with them.

This paper: arXiv:hep-ph/9502326v1, seems to have useful things to say about the issue.

I think the term "gluon condensate" usually means $\langle F_{\mu\nu} F^{\mu\nu}/4\rangle$. This is not really a condensate however, because it does not break any symmetry. Furthermore, since the operator $F_{\mu\nu} F^{\mu\nu}$ mixes with the identity operator under renormalization, it is a non-universal quantity, so it is not clear what it means physically.

There are various QCD "sum rules" that make use of this quantity, but they depend on a separation of "perturbative" from "non-perturbative" contributions. I've never understood how this can be done rigorously, and I am not alone in this skepticism, however the practitioners of this art claim that they know what they are doing, and I am not expert to argue with them.

This paper: arXiv:hep-ph/9502326v1, seems to have useful things to say about the issue.

I think the term gluon condensate" usually means $<F_{\mu\nu} F^{\mu\nu}/4>$. This is not really a condensate however, because it does not break any symmetry. Furthermore, since the operator $F_{\mu\nu} F^{\mu\nu}$ mixes with the identity operator under renormalization, it is a non-universal quantity, so it is not clear what it means physically.

I think the term gluon condensate" usually means $<F_{\mu\nu} F^{\mu\nu}/4>$. This is not really a condensate however, because it does not break any symmetry. Furthermore, since the operator $F_{\mu\nu} F^{\mu\nu}$ mixes with the identity operator under renormalization, it is a non-universal quantity, so it is not clear what it means physically.

There are various QCD "sum rules" that make use of this quantity, but they depend on a separation of "perturbative" from "non-perturbative" contributions. I've never understood how this can be done rigorously, and I am not alone in this skepticism, however the practitioners of this art claim that they know what they are doing, and I am not expert to argue with them.

This paper: arXiv:hep-ph/9502326v1, seems to have useful things to say about the issue.