Lounge Suits
Lounge suits are normal business suits, worn for semi-formal occasions with a shirt and tie.
The equivalent for women is a skirt or trouser suit, cocktail dress or evening gown depending on the time of day and the occasion.
A hat is also appropriate, but not essential, for women at such occasions as a wedding or the races.
If 'lounge suits' are stipulated on an invitation, it is fine to ask your host if you need a little more clarification.