Fr. bouton = button.
In medical terms used to refer to a condition of the fingers with a flexion of the proximal and hyperextension of the distal interaphalangeal joint (PIP and DIP). Usually found in rheumatoid arthritis or secondary to traumatic injuries. Outside of medicine boutonnière describes the arrangement of flowers in a button hole, commonly seen at wedding or funerals.
The opposite of a boutonnière is the swan neck deformity (hyperextended PIP with flexion in DIP)