What does "mannerism" mean?

Mannerism: Adherence to a peculiar style or manner; a characteristic mode of action, bearing, or treatment, carried to excess, especially in literature or art. Mannerism is pardonable,and is sometimes even agreeable, when the manner, though vicious, is natural . . . . But a mannerism which does not sit easy on the mannerist, which has been adopted on principle, and which can be sustained only by constant effort, is always offensive. Macaulay.