If you are using him/her in your imagination to enjoy sexual pleasure then it is lust.
The letter of law is that it separates sexual pleasure from union and procreation. The spirit of the law is basically that we should not reduce other people to the characteristics that we can benefit from. They should not be treated as an item that we use for an end. Sexually this means that, while we may adore a girl and acknowledge and admire her beauty as the fascinating other half of the image of God (personality, body and all,) we may not reduce her to her sexual capacities and use her in our imaginations to derive sexual pleasure because it robs her of her dignity and is a form of uncharitable objectification. I have noticed in my interior life that when a reduce a woman to her form, such a stranger in a photograph, it becomes quite easy to treat her like a cute sex toy. But something within me, the Natural Law you might say, feels a very strong aversion to thinking in such a way about someone I have met “in the flesh.” I start thinking about her feelings, how it would offend her and essentially make her feel raped by my mind. We should always adore the person: true love cares for their good, their happiness, their comfort and rights. True love seeks companionship, oneness, and in love making it desires to give oneself over to the spouse to please them and be closest to them.