In the introduction of a paper I was reading the author writes without elaborating that

**” In the case of a non-degenerate diffusion coefficient, Stroock and Varadhan , proved the existence of a unique weak solution for a SDE with bounded measurable drift , and bounded continuous diffusion coefficients. “**

So if the SDE is $ $ dX_t=\mu(X_t)dt+\sigma(X_t)dB_t $ $

So what does non-degeneracy of the diffusion coefficient mean here? Does it mean that $ \sigma(X_t)\sigma(X_t)^T$ is invertible for all $ t\ge 0$ almost surely? Or does it say something directly about the function $ \sigma$ ?