What is Big Mac Index?

Legal Definition
The Big Mac Index is published by The Economist as an informal way of measuring the purchasing power parity (PPP) between two currencies and provides a test of the extent to which market exchange rates result in goods costing the same in different countries. It "seeks to make exchange-rate theory a bit more digestible".

The index, created in 1986, takes its name from the Big Mac, a hamburger sold at McDonald's restaurants.
-- Wikipedia
Legal Definition
The index that measure purchasing power between currencies. The economist publishes the data in spring.