Someone who YOU find attractive. Someone who in your opinion is appealing. This person may not seem "Sexy" to others, but to you they are the definition OF sexy.
People saying that the girls in the example pictures aren't sexy, are not getting what sexy means. To you they may not be sexy, but, to someone they could be the most beautiful, sexy, hott thing EVER.
Boy 1- That girl isn't that hott dude. Look at the rolls..
Boy 2- So what man??? I mean look up top she got a nice rack and face... She seems sexy to me.