The ducat is the name of the monetary unit used in EU2. According to wikipedia, in the real world, "the ducat was a gold coin that was used as a trade currency throughout Europe before World War I."
Players often abbreviate it in two ways. Some use the dollar sign "$", since it has the right monetary connotation already, and there are no monetary units in EU2 other than ducats, so it is not confusing in context. Thus they would write "$100" for "100 ducats". Other players find the use of "$" confusing and so prefer to use a lowercase D. Thus they would write "100d" for "100 ducats". The EU2 wiki generally uses the lowercase D method.
Ducats are the unit used in many monetary contexts:
- for money, in a country's treasury.
- for the prices of things you can buy using treasury money.
- for base prices of goods, which figure into the computation of production and trade income.
- for the base tax value in provinces, which is used in several ways including the computation of province taxes.
- for the value of income streams, that is, monthly income and its components.
- for the amounts in investment streams created from monthly income via the spending sliders.
- for the "prices" of tech and stability increases that you buy with investment streams.