What is "formals"

Usage examples of "formals".

She has for you a variety of New Rome styles for women and men, a military uniform for Doctor Jacob, and one for Doctor Zebadiah, and evening formals for Doctor Elizabeth and Doctor Deety-and she's not sure where to send any of them.

I didn't believe for one second that a ship stocked like the Dora, run by identical redheads, could fail to have endless formals in green including this style or close to it.

The bellyband didn't work well under most formals, because you had yards of cloth to raise before you could get to the gun.

Looking around I see a few people dressed as I am, and a few dressed in long formals, tails almost sweeping the floor at their heels as they stand at the tables.