VMO - vastus medialis obliquus
It's the tear-drop shaped critter you see bulging out on the inner side of the thigh above the kneecap. It shows up very nicely in sprinters, they have honkin' VMOs.

Lots of VMO-specific exercises out there. When you find one that looks good to you, rest your fingers on the belly of the VMO to be sure you can feel it kicking in.

And ask someone to check your body mechanics to figure out what made you tight laterally in the first place. (anterior pelvic tilt, internally rotated hips, knee locking, over pronated feet, etc.) Wide hips allow women to get away with some sloppy mechanics, but we pay for it later.