A rotating disc whose surface is made from a Tungsten alloy that is bombarded with electrons from the filament, producing X-Rays.
This bombardment, in addition to generating X-Rays, releases a tremendous amount of heat.
The heat slowly migrates outward through the glass envelope and into the surrounding oil which carries it to the outside.
There is a bevel cut onto the target surface called the target angle. This angle determines the X-Ray beam spread and coverage.
The target is the main part of the anode assembly.
Targets can be made of solid metal or have a graphite backing.