From Wikipedia Definition Currency strength

Currency strength expresses the value of currency. For economists, it is often calculated as purchasing power,[1] while for financial traders, it can be described as an indicator, reflecting many factors related to the currency; for example, fundamental data, overall economic performance or interest rates.[2] It can also be calculated from currency in relation to other currencies, usually using a pre-defined currency basket. A typical example of this method is the U.S. Dollar Index. The current trend in currency strength indicators is to combine more currency indexes in order to make forex movements easily visible. For the calculation of these kind of indexes, major currencies are usually used because they represent up to 90% of the whole forex market volume.[3]