When I define myself as a tomboy, I definitely think of it as "a girl who likes to run with the boys." So, not butch, not androgynous, not menswear (men and boys are totally different to me) - definitely a physical girl who likes to enjoy (stereotypically) boyish activities. A bit of immaturity is key.
Since there are so many different types of "boys" out there, I think this will vary for each tomboy. For me, though, this means dressing for sort of rough-and-tumble activities and wearing boyish styles without obscuring the feminine shape. Short pants, jeans, graphic tees with Star Wars or comic characters, rugged boots, utility jackets. I DO wear heels and carry handbags, but always ones that I can still run around and be relaxed/comfortable in.
Again, for me, it's not about *looking* like a boy, but about being ready to *play with* the boys. Fishin' in the creek? Pinball at the arcade? Climbin' a treehouse? Shootin' cans in the desert? Buildin' a robot? Let's go! I've got my trusty slingshot in my back pocket.
A tomboy is NOT prim, prissy, ruffled, coiffed, delicate, dainty, waiflike, or wide-eyed. She isn't afraid to get a little dusty. She doesn't blush at salty language or faint at the first sign of trouble. If you bully her, she'll probably sock you back rather than cry. She'd rather snuggle up with the family hound than cuddle a baby doll.
For celebrity examples, I definitely don't see Katherine Hepburn. To me, she is an example of menswear style - definitely very mature, pristine, and elegant. I think more Angelina Jolie (back when she was married to Billy Bob), Ellen Paige, even Gwen Stefani (she rocks out with the boys while definitely looking like a glam gal).