Bouquet.

Bouquet - an arrangement of flowers carried at a ceremony.

After the reception the bride throws her bouquet back over her shoulder where the unmarried female guests and bridesmaids group together. Tradition holds that the one who catches the bouquet will be the next one of those present to marry.
A parallel custom is for the groom to remove the garter worn by the bride and throw it back over his shoulder toward the unmarried male guests. Again the one who catches it will be the next to marry.

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Image: Sharron Goodyear / FreeDigitalPhotos.net