I am referring to the value of the currency compared to itself in the past. A dollar today is not worth the same amount as a dollar yesterday. Inflation is that

I am referring to the value of the currency compared to itself in the past. A dollar today is not worth the same amount as a dollar yesterday. Inflation is that decline in value over time. Less commonly, it is also possible for a currency to become worth more over time, which is what we refer to as deflation. If, for argument's sake, a dollar halved in value, the price of a widget of constant value would double to maintain equality in the exchange. What makes things complicated is that the value of the widget isn't constant and can also change over time, so we can't just look at that widget to see how the value of the currency is changing. Instead, we try to look for price trends across a wide range of products (CPI) to try and infer what is related to currency devaluation rather than changing values of other goods and services.