>Lust is disordered desire for or inordinate enjoyment of sexual pleasure. Sexual pleasure is morally disordered when sought for itself, isolated from its procreative and unitive purposes.
>CCC 2351
You lust when you think of a woman, or look at one, regardless of the level of clothing and consciously think that you would like to have sex with her or you imagine having sex with her or deliberately arouse yourself.
It's definitely not a sin to think that someone looks good. It's not even a sin to admire certain body parts specifically, but if you have problems with sexual arousal and the sins that stem from it then you'd better avoid it for safety's sake