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Kylie Jenner has hit the headlines as the youngest self-made billionaire, but what actually is a billionaire?

That sounds like a simple question - obviously it's someone who has a billion pounds, dollars, euros etc in the bank, right? Well that's not actually the case.

Although billionaires will no doubt have a healthy bank balance, they're very unlikely to have access to that much money when they check their bank balance.

Their financial value - or net worth if you want to use a bit of business language - includes not just the money they physically have, but the value of things like houses, cars, and other assets. The net worth also takes into account the value of a business someone owns when figuring out how much they're worth. All of this counts towards what makes someone a billionaire.

Despite having lots of types of income, Kylie's cosmetics business is the main source of her entry onto the billionaires list. Her company is estimated to have had sales worth$360m last year alone.

What makes a billion is also different depending on where you are; in the UK we refer to a billion as a million million, but in the US it's a thousand million.

Although the numbers are different in each place, the standard we use to define a billionaire is the American way - someone worth more than 999,999,999 in the local currency.