What is Cap Of Maintenance?

Legal Definition
A cap of maintenance, known in heraldic language as a chapeau gules (French: "red hat"), is a ceremonial cap of crimson velvet lined with ermine, which is worn or carried by certain persons as a sign of nobility or special honour. It is worn with the high part to the fore, the tapering tail behind. It may substitute for the torse in the heraldic achievement of a person of special honour granted the privilege by the monarch. It thus appears in such cases on top of the helm and below the crest. It does not, however, feature in the present royal arms of England, which show the royal crest upon the royal crown, itself upon the royal helmet.
-- Wikipedia
Legal Definition
One of the regalia or ornaments of state belonging to the sovereigns of England, before whom it is carried at the coronation and other great selemnities. Caps of maintenance are also carried before the mayors of several cities in England. Enc. Lond.
-- Black's Law Dictionary