NOUN (2)

1. the brother of your father or mother; the husband of your aunt;

2. a source of help and advice and encouragement;
- Example: "he played uncle to lonely students"

The Collaborative International Dictionary of English v.0.48:

Uncle \Un"cle\, n. [OE. uncle, OF. oncle, uncle, F. oncle, fr.
   L. avunculus a maternal uncle, dim. of avus a grandfather;
   akin to Lith. avynas uncle, Goth. aw? grandmother, Icel.
   [=a]i great grandfather.]
   1. The brother of one's father or mother; also applied to an
      aunt's husband; -- the correlative of aunt in sex, and of
      nephew and niece in relationship.
      [1913 Webster]

   2. A pawnbroker. [Slang] --Thackeray.
      [1913 Webster]

   3. An eldery man; -- used chiefly as a kindly or familiar
      appellation, esp. (Southern U. S.) for a worthy old negro;
      as, "Uncle Remus." [Colloq.]

            Plain old uncle as he [Socrates] was, with his great
            ears, -- an immense talker.           --Emerson.
      [Webster 1913 Suppl.]

   My uncle, a pawnbroker. [Slang]

   Uncle Sam, a humorous appellation given to the United
      States Government. See Uncle Sam, in Dictionary of Noted
      Names in Fiction.
      [1913 Webster]

WordNet (r) 3.0 (2006):

uncle
    n 1: the brother of your father or mother; the husband of your
         aunt [ant: aunt, auntie, aunty]
    2: a source of help and advice and encouragement; "he played
       uncle to lonely students"

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "uncle":
   aunt, auntie, blood brother, brethren, brother, bub, bubba, bud,
   buddy, country cousin, cousin, cousin once removed,
   cousin twice removed, daughter, father, first cousin,
   foster brother, frater, grandnephew, grandniece, granduncle,
   great-aunt, great-uncle, half brother, kid brother, mother, nephew,
   niece, nuncle, nunks, nunky, second cousin, sis, sissy, sister,
   sister-german, sistern, son, stepbrother, stepsister, unc, uncs,
   uterine brother



V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

UNCLE
       Unix Net for Computer security in Law Enforcement (org., USA,
Unix), "U.N.C.L.E."