Brightness is not the correct terminology for this control but decades of using this terminology will be difficult to change. It is a black level control and determines how black you can make black without burying the darker areas of the image.
Contrast acts like a brightness control and has also been labeled as picture by some manufacturers. Contrast still fits though as you are adjusting the difference in light output between the bright parts of the image and the dark parts.
In most cases adjusting one will throw off the other so always double check them.
CRT Brightness
Provide your black level pattern and adjust accordingly per the most common viewing environment used by the client. With CRT RPTV displays it is best to bump up the brightness just a couple of clicks to offset wash out due to light reflecting off the screen and bouncing around inside the cabinet making it