An email digest is an email that is automatically generated by an electronic mailing list and which combines all exchanged emails during a time period (e.g. day, week, month, etc.)
Email digests are currently available as an opt-in feature in electronic mailing list systems such as GNU Mailman or LISTSERV.
It is called an abridged summary in Google Groups.
Other systems than mailing lists implement such a feature.
YouTube compiles all the communications to users within a time period into a single email.