The one who will be fully responsible for the project given to him/her. The term was furnished from Apple meeting, it says that there will be one individual who'll be responsible for an agenda or meeting all the criterion for that specific project. This technique has been adopted by many of the leading companies appointing DRI in their product management departments.
Diagrams
Originated from/Coined by
Apple Inc.
Further Reading/ References
https://about.gitlab.com/handbook/people-operations/directly-responsible-individuals/