Anti-roll bar-- (a.k.a. sway bar, stabilizer bar) a bar typically 3/4" to 1-1/8" in diameter connecting the right and left side suspension components across the front pair or rear pair of wheels. The bar may be straight or have several bends in it to route its way from one side of the car to the other. The bar will have some capacity for flexing

The purpose is to add resistance to body roll. Body roll by itself is not a bad thing, but it tends to alter the relationship of the suspension geometry to the road surface. This in turn alters the angle of the wheel and tire on the road surface. The purpose of limiting body roll is to maintain as close as possible the intended relationship of the tire to the road surface.

A secondary function of the anti-roll bar is to alter the roll stiffness differential between the front and rear of the car. This is an important element used to balance the tire grip between the front and rear of the car and tune the handling relative to understeer and oversteer characteristics.