A somewhat derogatory phrase for a portrait
that does little more than attempt to make the subject
appear in the most attractive
light as possible.
You see, a real portrait is introspective - it examines the personality of the subject and attempts to place them within the most appropriate light. It delves into the psyche and if done well finds a truer you than you may have thought existed all along. Like many aspects of photography a portrait is super-real. It's detached from reality by dint of being so much more than reality ever could be. It tells you so much more than real life possibly could.
Not so with a headshot. All anyone wants out of a headshot is to look pretty. Outside of an attempt to get a job as an actor/model/etc they serve no purpose. The subject desires nothing more than a simple representation so that they're remembered the day following an audition. The photographer desires nothing more than the money provided for the effort.
Neither of these things are bad. They're necessary evils in a world where we can't always get what we want from the get go.