These are sources that index, abstract, organize, compile, or digest other sources. Some reference materials and textbooks are considered tertiary sources when their main purpose is to list or summarize other information. Tertiary sources are usually not credited to a particular author.
Morehouse, Sarah. (2012, July 2). Primary, secondary, and tertiary sources. [Video]. YouTube. https://www.youtube.com/watch?v=bhNyTIcszhA&t=6s