Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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