A deficit occurs when there isn't a sufficient amount of money to cover all of the expenses and debts or when you are not as good at something as you should be. (noun)
An example of a deficit is when you owe $100 and only have $90.
An example of a deficit is when a child in 9th grade reads at a 4th grade level.
See deficit in Webster's New World College Dictionary
noun
the amount by which a sum of money is less than the required amount; specif., an excess of liabilities over assets, of losses over profits, or of expenditure over income
See deficit in American Heritage Dictionary 4
(dĕfˈĭ-sĭt)
noun
a. Inadequacy or insufficiency: a deficit in grain production.
b. A deficiency or impairment in mental or physical functioning.
c. An unfavorable condition or position; a disadvantage: rallied from a three-game deficit to win the playoffs.
a. The amount by which a sum of money falls short of the required or expected amount; a shortage: large budget deficits.