The Collaborative International Dictionary of English v.0.48:

Fraternity \Fra*ter"ni*ty\, n.; pl. Fraternities. [F.
   fraternit['e], L. fraternitas.]
   1. The state or quality of being fraternal or brotherly;
      brotherhood.
      [1913 Webster]

   2. A body of men associated for their common interest,
      business, or pleasure; a company; a brotherhood; a
      society; in the Roman Catholic Church, an association for
      special religious purposes, for relieving the sick and
      destitute, etc.
      [1913 Webster]

   3. Men of the same class, profession, occupation, character,
      or tastes.
      [1913 Webster]

            With what terms of respect knaves and sots will
            speak of their own fraternity!        --South.
      [1913 Webster]

   4. A social club for male college undergraduates. They often
      have secret initiation rites, and are named by the use of
      two or three Greek letters. The corresponding association
      for women students is called a sorority.

   Syn: frat.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

fraternity
    n 1: a social club for male undergraduates [syn: fraternity,
         frat]
    2: people engaged in a particular occupation; "the medical
       fraternity" [syn: brotherhood, fraternity, sodality]

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "fraternity":
   affiliation, agnation, alignment, alliance, amalgamation, ancestry,
   association, blood, blood relationship, boon companionship,
   brotherhood, brotherliness, brothership, cahoots, camaraderie,
   chumship, circle, clan, clannishness, clique, closeness, club,
   coadunation, coalescence, coalition, cognation, colleagueship,
   collegialism, collegiality, combination, common ancestry,
   common descent, community, community of interest, companionship,
   company, comradeship, confederacy, confederation, confraternity,
   confrerie, connection, consanguinity, consociation, consolidation,
   consortship, cooperation, copartnership, copartnery, coterie,
   country club, cousinhood, cousinship, crowd, enation,
   esprit de corps, fatherhood, federation, fellowship, filiation,
   fraternal order, fraternalism, fraternization, freemasonry,
   friendship, fusion, guild, hookup, inclusion, incorporation,
   integration, kindred, kinship, league, maternity, matrilineage,
   matriliny, matrisib, matrocliny, membership, merger, motherhood,
   order, partaking, participation, partnership, paternity,
   patrilineage, patriliny, patrisib, patrocliny, propinquity,
   relatedness, relation, relationship, secret society, set, sharing,
   sibship, sisterhood, sisterliness, sistership, society, sodality,
   solidarity, sorority, tie-in, tie-up, ties of blood, unification,
   union, unity