The Collaborative International Dictionary of English v.0.48:

Colloquial \Col*lo"qui*al\, a. [See Colloqui.]
   Pertaining to, or used in, conversation, esp. common and
   familiar conversation; conversational; hence, unstudied;
   informal; as, colloquial intercourse; colloquial phrases; a
   colloquial style. -- Col*lo"qui*al*ly, adv.
   [1913 Webster]

         His [Johnson's] colloquial talents were, indeed, of the
         highest order.                           --Macaulay.
   [1913 Webster]

WordNet (r) 3.0 (2006):

colloquial
    adj 1: characteristic of informal spoken language or
           conversation; "wrote her letters in a colloquial style";
           "the broken syntax and casual enunciation of
           conversational English" [syn: colloquial,
           conversational]