The Collaborative International Dictionary of English v.0.48:

Deficit \Def"i*cit\, n. [Lit., it is wanting, 3d person pres.
   indic. of L. deficere, cf. F. d['e]ficit. See Defect.]
   Deficiency in amount or quality; a falling short; lack; as, a
   deficit in taxes, revenue, etc. --Addison.
   [1913 Webster]

WordNet (r) 3.0 (2006):

deficit
    n 1: the property of being an amount by which something is less
         than expected or required; "new blood vessels bud out from
         the already dilated vascular bed to make up the nutritional
         deficit" [syn: deficit, shortage, shortfall]
    2: a deficiency or failure in neurological or mental
       functioning; "the people concerned have a deficit in verbal
       memory"; "they have serious linguistic deficits"
    3: (sports) the score by which a team or individual is losing
       [ant: lead]
    4: an excess of liabilities over assets (usually over a certain
       period); "last year there was a serious budgetary deficit"

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "deficit":
   absence, arrear, arrearage, arrears, back debts, back payments,
   balance, beggary, bouncing check, break, credit, decline,
   defalcation, default, defect, defectiveness, deferred payments,
   deficiency, deficit financing, delinquency, deprivation,
   destitution, difference, discontinuity, discrepancy, dollar gap,
   drought, epact, failure, falling short, famine, gap, hiatus,
   imperfection, impoverishment, inadequacy, incompleteness,
   inferiority, insufficiency, interval, lack, lacuna, loss,
   missing link, need, net, omission, outage, overdraft, remainder,
   scantiness, short measure, shortage, shortcoming, shortfall, slump,
   starvation, surplus, ullage, underage, want, wantage



Bouvier's Law Dictionary, Revised 6th Ed (1856):

DEFICIT. This Latin term signifies that something is wanting. It is used to
express the deficiency which is discovered in the accounts of an accountant,
or in the money in which he has received.