From WordNet (r) 1.6 [wn]:

  reference
       n 1: a remark that calls attention to something or someone; "she
            made frequent mention of her promotion"; "there was no
            mention of it"; "the speaker made several references to
            his wife" [syn: {mention}]
       2: a short note acknowledging a source of information or
          quoting a passage; "the student's essay failed to list
          several important citations"; "the article includes
          mention of similar clinical cases" [syn: {citation}, {credit},
           {mention}, {quotation}]
       3: an indicator that orients you generally; "it is used as a
          reference for comparing the heating and the electrical
          energy involved" [syn: {reference point}, {point of
          reference}]
       4: a book to which you can refer for authoritative facts; "he
          contributed articles to the basic reference work on that
          topic" [syn: {reference book}, {reference work}, {book of
          facts}]
       5: a formal recommendation by a former employer to a potential
          future employer describing the person's qualifications and
          dependability; "requests for character references are all
          to often answered evasively" [syn: {character}, {character
          reference}]
       6: the class of objects that an expression refers to; "the
          extension of `satellite of Mars' is the set containing
          only Demos and Phobos" [syn: {denotation}, {extension}]
       7: the act of referring; "reference to an encyclopedia produced
          the answer" [syn: {consultation}]
       8: a publication (or a passage from a publication) that is
          referred to; "he carried an armful of references back to
          his desk"; "he spent hours looking for the source of that
          quotation" [syn: {source}]
       9: the relation between a word or phrase and the object or idea
          it refers to; "he argued that reference is a consequence
          of conditioned reflexes"
       v : refer to [syn: {cite}]

In Xanadu documents exist as referential structures where documents and everything represented in their structure are managed as references, right down to the letter.