Oneitis is the girl you obsess about. Hardcore AFCs become afflicted with 1itis with a really attractive girl they've never even talked to. It's an obsession bordering on stalking.
1itis can also occur with your current LTR or even ex-. It's not always a bad thing, but it can become unhealthy if your life, pursuits, and social circle start and end with her.
Also not recommended if she's your first as you have nothing, physically or emotionally, to compare it to in order to know if it's "appropriate" to get all 1itis/LTR with her.