There are many names for it - virtual water, embedded water, embodied water, hidden water - but the concept is the same.
It is a calculation of how much water is used in the production and trade of food and consumer goods.
The idea of a water footprint was first created by Professor John Allan from King's College in London, who received the 2008 Stockholm Water Prize for his efforts.
For example, production of 1kg of wheat 'costs' 1,300 litres of virtual water while 1kg of beef costs around 15,000 litres.
'The water is said to be virtual because once the wheat is grown, the real water used to grow it is no longer actually contained in the wheat,' Allan said. 'The concept of virtual water helps us realise how much water is needed to produce different goods and services.'
In order to calculate the water cost of a product, it is necessary to break down each step of production.