A phantom pregnancy is when someone thinks they're pregnant, when they're actually not. Sort of like a phantom text, when you think you feel your phone vibrate, but it really didn't. You begin to imagine signs of pregnancy, such as morning sickness, irritation towards everything and everyone, the munchies, ect.
Girlfriend: I'm pregnant! I swear I'm pregnant!
Boyfriend: .. That's not possible.
Girlfriend: Yes it is! We've screwed a thousand times!
Boyfriend: It's probably just a phantom pregnancy...
Girlfriend: No! I swear, I am! I think I feel kicking too!
Boyfriend: I'm sterile.