What defines a situationship?

It does technically sound like what you have going on is a situationship, which is, by definition, "a casual relationship that isn't committed or established." Usually the term is applied to people who have no intention of committing and are just interested in stringing someone along, until they find somebody better or get bored. I don't know if that applies to your guy, but if you haven't had any discussions with him about where your relationship is going and, more importantly, he seems unwilling to have those discussions, you might consider taking a step back and thinking through if this is the sort of relationship you want, where you're wanting more and the other person seems fine with how things are going. Some people are fine with dragging someone else's heart around for long periods of time without any intention of committing to them.

If you find yourself being intimate with someone (emotionally and/or sexually) and consistently spending time with them without ever having labeled the relationship...then you're in a situationship. If that's what you want, then that's great for you! But if you want something more out of the relationship, you better have an open conversation with him

It can be easy to confuse a situationship with the beginnings of an actual relationship because there's a lot of ambiguity and feeling things out in both situations. I think the difference is, if you try to talk to him about getting into a serious relationship and he dodges the question or gets distant, then it's likely a situationship...

If you'd like things to go further make sure you communicate with him! If you let him keep the ball in his court and he just wants you as a hook up, then you'll never get what you want