To me, alterous is a type of love where you don't care what the label of your relationship is.
If you have a platonic attraction to someone, you want to be their friend. If you have romantic attraction, you want to be their lover. With sexual attraction, you want to be their sexual partner, with sensual attraction, their cuddle buddy. And if the person you're attracted to feels a different way, it can be painful (for example, being in love with someone who'd rather be "just friends" can be heartbreaking).
With alterous attraction, there are multiple of the above that you'd be okay with being called. If they want to be friends, great! And if one day they ask you out on a date, let's go! They want to cuddle, or even have sex? Anything for you! (Note that if you're repulsed by sex, romance, or touch, some of these might not apply to you.)
As long as you get to have a close, trusting relationship with the person you're alterously attracted to, you're happy.