excursus

from WordNet (r) 3.0 (2006)

excursus
    n 1: a message that departs from the main subject [syn:
         {digression}, {aside}, {excursus}, {divagation},
         {parenthesis}]

from The Collaborative International Dictionary of English v.0.48

Excursus \Ex*cur"sus\, n. [L., fr. excurrere, excursum. See
   {Excurrent}.]
   A dissertation or digression appended to a work, and
   containing a more extended exposition of some important point
   or topic.
   [1913 Webster]