pocket square; decorative handkerchief
A decorative handkerchief placed in the breast pocket of a suit jacket as a fashion accessory. Wasei-eigo from English 'pocket handkerchief'.
wasei-eigo (Japanese-coined English), from pocket (handker)chief
Putting a pocket square in the breast pocket of a suit makes you look stylish.
At weddings, a white pocket square is often used.