I've never heard anyone refer to a suit worn to formal hall as a lounge suit before. Sounds far posher, but from googling, it looks like it's basically a normal suit.
But as rawjoh1 said, basically half formal, although I doubt you'd get refused entry if you went for less ('tho you might feel a bit out of place/waste an opportunity for playing dressing up). For men it's regular suits (with ties, usually), for women, more varied, but basically as said ^^.