What is a fascinator?

A "fascinator" is an ornamental headdress or headband.

The signal took instant effect. The musicians hurried to their instruments, the dancers—some already half-muffled for departure—fell into line down each side of the room, the older spectators slipped back to their chairs, and the lively young man, after diving about here and there in the throng, drew forth a girl who had already wound a cherry-coloured "fascinator" about her head, and, leading her up to the end of the floor, whirled her down its length to the bounding tune of a Virginia reel.