For my purposes, loungewear and pajamas are the same thing.
I consider sweatpants, yoga pants, t-shirts, sweatshirts, etc. to be athleisure wear, not loungewear. That is what I put on when I get home from work. From there I change into my pajamas.
But I guess since I lounge around the house in my athleisure wear, it could be considered loungewear. I think of loungewear though as nicer pajama bottoms and matching top or a caftan, not a t-shirt and shorts.