When something that should be there, isn't. Or, it is there, you just can't see it. Invisible things can still be touched, invisible people can still be fucked. Invisible things have a habit of going missing, so if I were you, I'd ask for a visible baby when you request one from your spouse. Scientists once argued whether the insides of humans were invisible. It took 4 years and 400 billion English pounds of research to establish that no, people innards are not invisible. Something that was discovered, however, was just how much blood a body can lose before it still doesn't lose visibility. Invisibility! Look out for it!