This is a Dynalite #4003 UV Coated Flashtube with frosted dome. Most current, professional flash heads and monolights are equipped with user replaceable flashtubes that may simply be plugged into, and unplugged from their sockets. With proper handling, these flashtubes may last for tens of thousands of flashes under normal use.
The glass dome provides protection from accidental contact with fingers and equipment. The diffusion slightly softens the light output from the flashtube.