A deficit is a situation in which the difference between revenues and expenditures is negative, i.e., expenditures exceed revenues.
Generally, when we speak of deficit, we refer to the situation of a country or a local administration. In the case of governments, the deficit, in addition to being expressed in monetary units, can be expressed as a percentage of GDP, thus showing the importance it has in relation to the economy of the country to which it refers.
The opposite situation to the deficit is the surplus, which occurs when income exceeds expenditure over a period of time.
If you are interested in seeing the evolution of the deficit in the different countries, you can see it in Ranking deficit by country