Is sad stand for seasonal affective disorder or social anxiety disorder?

Either works: Honestly, I have heard it referred to either. You could also throw in a mix of "separation anxiety disorder" when talking about kiddos as well.

SAD: SAD is now referred to as Depressive Disorder with Seasonal Pattern in DSM 5. It is a subtype of depression. I have not heard of Social Anxiety Disorder (Social Phobia) referred to as SAD. The problem with using abbreviations is that it can cause a big mix up. Such as using BPD for borderline personality disorder & bipolar disorder.