I'm reading the "Personal MBA" book right now and it says that "Yoga is $8 billion industry worldwide". What does it mean? And how can one calculate the amount of money in some industry?
Considering the economic of world as a whole it could simply refers to that total spending by people on yoga might be \$8 billion. As in an economy total spending is the total revenue of a sector. For this particular scenario it simply means that yoga as an economic variable have transaction of \$8 billion. Regarding the calculation we can de assured that no exact and absolute calculation could be made on national level how could it be calculated exactly at global level. However the estimated aggregated spending on yoga sector nation could be determined. Which further on processing might result to provied aggregate global spending on yoga as a figure, which is tentative not exact figure.
It's ambiguous - that is, the term is used to mean several different things.
It could mean that \$YYY is the annual turnover of the industry. Or the total market capitalisation of companies in the industry. Or possibly the annual investment in the industry.
In your particular context, of "Yoga is \$8 billion industry worldwide", I'd expect it probably refers to the annual turnover of the industry
My understanding is it is the estimate of total revenue firms in the industry could potentially make. Analysts come up with the total estimate by considering the demand for the service or product in the industry and price ranges at which the product/service would be sold.
It depends. For example:
a report about crowdfunding from the World Bank refers to amount of investment:
Crowdfunding based initially on soliciting pledges or donations has emerged as a multibillion-dollar global industry. [...] Projections for the size of the crowdfund investing market range from US\$3.98 billion (Best, Neiss, Stralser, and Fleming 2013) to as much as US\$300 billion over the coming years depending on the level of enabling regulation adopted by governments (Huessner 2012).
This report by World Bank about agribusiness in Africa refers to "Value added and household expenditure" (Box 3.1, page 15) when they say:
Agriculture and agribusiness together are project ed to be a US\$ 1 trillion industry in Sub-Saha- ran Africa (SSA) by 2030 (compared to US\$ 313 billion in 2010)
this article about cancer refers to a billion dollar industry in terms of:
[i]n 2012, the reported spending on cancer treatment was 124.6 billion dollars.
Perhaps more oddly, this article refers to pollution as a billion dollar industry in terms of the economic costs it imposes to the economy:
For example, the United States is estimated to lose \$45 billion every year due to air pollution, while the UK loses \$7.6 billion annually. Germany comes in at \$18 billion
In summary, there is no universal agreement on what that is supposed to mean. As the examples above show, it depends on the context.