Girls that have interests associated with men like cars, video games, technology, and science or sports and have an attitude that is conducive to hanging out with dudes that most women just don't or can't. They can have the short hair or fashion sense of men, but I think thay over complicates it since you are confusing aesthetics with the core definition. I've seen tomboys that dress up like girls for the most part but ate very clearly tomboys. It's more a label about the girl's personality rather than a particular fashion statement, although women that dress up like your average teenage or 20 something year old dude tend to be tomboys in some way shape or form in one way or the other.
For a similar reason, sexuality has nothing to do with it and this is why most people seem to think tomboys are lesbians when they probably are straight. All the ones I met IRL were just regular girls who were sexually attracted to boys but also could hang out with them and understand them to be friends to hang out with IRL.