n. 附记, 补说, 附录, 离题话

[ noun ]
a message that departs from the main subject
<noun.communication>

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.