When your bust, waist and hips are relatively even.
The trick with this body shape is adding attention or mass to your shoulders or hips. Crewnecks and turtlenecks are perfect for creating a solid line of fabric from shoulder to shoulder that will accentuate your shoulders. Want to show off a bit more? Strapless, halters, and scoop necks are all your friends! Tuck your top in when possible, either French or full, and belt whenever possible. Drawing attention to your waist and exaggerating the shoulder or hip will make your waist more petite than your other measurements. Regarding pants, your choices are wide open, but my best recommendation is higher-waist pants with slash pockets to exaggerate the hip.
A-line cuts will be your best bet when it comes to dresses, but if you prefer something more fitted like the Ashley, go with a longer sleeve and, if possible, belt the waist. The longer sleeve will add volume to the upper half of your body, and the belt will define your waist.