What is a masthead light on a vessel?

Rules of the Road

Lights & Shapes, Rule 21-International

Definitions

(a) "Masthead light" means a white light placed over the fore and aft centerline of the vessel showing an unbroken light over an arc of the horizon of 225 degrees and so fixed as to show the light from the right ahead to 22.5 degrees abaft the beam on either side of the vessel.