U.S. Dollar Index, or USDX, is an index, or a measure of the
value of the U.S. dollar relative to majority of its most significant
trading peers. USDX goes up when the US dollar gains "strength"
(value) when compared to other currencies.
The index is calculated by factoring in the exchange rates of six major world currencies: the euro, Japanese yen, Canadian dollar, British pound, Swedish krona and Swiss franc.
It dates back to 1973, started with a base of 100 and is relative to this base. This means that a value of 120 would suggest that the U.S. dollar experienced a 20% increase in value over the time period.
USDX can be traded as a futures contract on the IntercontinentalExchange (ICE). It is also available in exchange-traded funds (ETFs), options and mutual funds.