The Collaborative International Dictionary of English v.0.48:

Tacit \Tac"it\, a. [L. tacitus, p. p. of tacere to be silent, to
   pass over in silence; akin to Goth. [thorn]ahan to be silent,
   Icel. [thorn]egja, OHG. dag[=e]n: cf. F. tacite. Cf.
   Reticent.]
   Done or made in silence; implied, but not expressed; silent;
   as, tacit consent is consent by silence, or by not
   interposing an objection. -- Tac"it*ly, adv.
   [1913 Webster]

         The tacit and secret theft of abusing our brother in
         civil contracts.                         --Jer. Taylor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tacit
    adj 1: implied by or inferred from actions or statements; "gave
           silent consent"; "a tacit agreement"; "the understood
           provisos of a custody agreement" [syn: silent, tacit,
           understood]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "tacit":
   assumed, implicit, implied, inarticulate, inferred, intimated,
   silent, suggested, taken for granted, undeclared, understood,
   unexpressed, unsaid, unspoken, unstated, unuttered, unvoiced,
   wordless



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

TACIT. That which, although not expressed, is understood from the nature of
the thing, or from the provision of the law; implied.