There are some great, very useful threads on here about definitions, etc. Look for the stickies at the top of each of the various fora here.
But you don't really need definitions from outside sources nearly as much as you need some clear, open and honest communication with this other person.
Be kind and loving, but be clear. Find out how s/he sees your relationship, and express your view of it.
Here's one possibility for an opener, if you're having trouble getting started: "It seems to me that you and I might not be on the same page about our relationship. You are really important to me and I need some clarity around this, so can we talk about it?"