1. wandering from the main path of a journey

Synonym: excursion

2. a turning aside (of your course or attention or concern)

e.g. a diversion from the main highway
a digression into irrelevant details
a deflection from his goal

Synonym: diversiondeviationdeflectiondeflexiondivagation

3. a message that departs from the main subject

Synonym: asideexcursusdivagationparenthesis