Dictionary.net

lose clarity or turn aside esp. from the main subject of attention or course of argument in writing, thinking, or speaking; "She always digresses when telling a story"; "her mind wanders"; "Don't digress when you give a lecture"

lose clarity or turn aside especially from the main subject of attention or course of argument in writing, thinking, or speaking; "She always digresses when telling a story"; "her mind wanders"; "Don't digress when you give a lecture"

Stray, digress. Hence divagation n. [Latin]