From Latin 'excursio' (a running out) — originally a military sortie, now a short trip from a base to which one returns.
A short journey or trip, especially one taken for leisure or a particular purpose; a deviation from a regular or intended course.
From Latin 'excursio' (a running out, a sortie, a digression), from 'excurrere' (to run out, to sally forth), composed of 'ex-' (out) and 'currere' (to run). The Latin military sense was 'a sortie' — a sudden running out from a fortified position to attack. The rhetorical sense was 'a digression' — running out from the main line of argument. English kept both the military and the leisure senses, though the leisure sense (a pleasure trip, a running-out for enjoyment) now dominates. Key
In academic writing, an 'excursus' (plural 'excursus' or 'excursuses') is a detailed digression within a longer work — a running-out from the main argument to explore a side topic at length. It is the scholarly cousin of the pleasure excursion: instead of running out for a day at the beach, the author runs out for a few pages on an interesting tangent.