to start to talk about something that is not connected with the main point of what you are saying (Oxford)
to move away from the main subject you are writing or talking about and to write or talk about something else (Cambridge)
To digress for a moment, I would like to remind you that there will be no classes next week. (Oxford)
The lecturer temporarily digressed from her subject to deal with a related theory. (Cambridge)
digressed for a moment
lecturer temporarily digressed
Deviate
Diverge
Keep/stick to the point
On Coarse
Digression