Love.
An abstract noun, denoting a 'great liking or affection', a 'feeling of the heart', 'sexual affection or passion', 'a loved person, a sweetheart' and even a tennis score: 'no score; nil'.
Sometimes it is a verb. To love someone, to be in love (defined as something stronger than normal love by his dictionary), to 'feel love for a person or thing'.
The word, in any of its forms, even with the surreptitiously added prefix of 'in', doesn't even begin to do justice to how he feels for her. Not even the connotations come close, but it's the only word he has. Labelling it is perhaps wrong, like trying to define what should always remain unknown, trying to capture something that should never be examined.
To him, she is the most beautiful thing to ever walk the planet; hers or any other. They say love is blind, and perhaps he is biased, but he thinks perhaps he doesn't need much help with this one.
Every day it amazes him how someone so ordinary, a 21st century shop-girl overlooked by so many, can be so wonderful, and every day she finds new ways to astound him. She has more power over him than anything else he has ever known. Guilt, loneliness, regret…she drives them all away, blows them all out of the water. Nothing but her can enter his mind when she is around, and he is glad of it.
He isn't supposed to care for her so much, to be so dependent on her presence. He never intended to fall for her. If the other Time Lords were still around, he would not have been allowed to feel this way about her, but he does.
He isn't quite sure when the universe became theirs, but it is.