Simply put; your one and only, the one you're destined to be with, the love of your life, the one you can't live without, your other half, etc.
It's the love that survives anything and everything, that struggles and pulls through, that even when it seemed hopeless and out of control it still beat strongly. It's the love that is not superficial, a love that transcends not only the physical world but time itself. It's not perfect, but it's wonderful and still exhilarating after all those years. There are people who get married, then get divorced, but come back to one another *ten years later*. A love that isn't entirely human. It's the love that, if lost by a foil against destiny, will leave the people empty, pinning and never forgetting and never fulfilled. It's the love that makes you cringe at the thought that, had your life gone differently even for a millisecond, you may have not met this person - but in your heart you know you would have, some way, somehow, because soulmates always find one another.
I feel like we've met a million times before in our past lives, that every tiny step of our lives led us directly to one another. He's my soulmate.