Per capita
  
Per capita just means "per head." Yep. Comes from Latin, like oh so many things: salsa, dressing, and dancing, among many others.
The word decapitation comes to, uh...mind. Like, what happens when those flying human taxi drones tip too far to the left, or when King Henry got tired of his Queen.
So, if that happened often, it’d throw off the per capita calculations. And same deal if we suddenly had a lot of two-headed people being born, like that guy in Men in Black 2.
Per capita is a useful metric in a bunch of financial calculations. A common set is GDP, or Gross Domestic Product, per capita. Like, you can have a ton of GDP...say, 10 trillion dollars from your country, WhateverStan. If you have only a million people, that’s a ton of production productivity. But if you’re, say, China, with well over a billion people, then it’s a much lower GDP per capita.
The calculations get quoted when talking about cell phone monthly subscriptions computer ownership, or a particularly rollicking house party.
So yeah, the notion of per capita is pretty important. But don't stress if you're not an expert yet. It's nothing to lose your head over.