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]

WordNet (r) 3.0 (2006):

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