It’s called…a plank. What is a plank? Quite simply, it’s an isometric exercise for the abs (where you perform one action, holding still for the entire duration of the move). You may be learning the plank exercise for the first time or you may already use it regularly in your workouts. But guess what? Most trainers consider the plank to be one of the most effective abs exercises.
Not only does it strengthen and firm your abs, but also works all the core muscles — the back, hips, etc
Article courtesy of: http://blackdoctor.org/3494/the-best-ab-exercise/2/