I had a discussion with my friend once about brain fog, and I said brain fog wasn't a real symptom of depression; it was just sort of a layperson's description of cognitive difficulties. Brain fog itself wasn't exactly real, per se; I said I wasn't exactly wrong about that, but I wasn't exactly right either. Brain fog is not exactly a medical descriptor, but I identify with it as a part of my illness(es). I can now attest to the fact that brain fog is real.