Tertiary sources are publications that summarize and digest the information in primary and secondary sources to provide background on a topic, idea, or event.
These are sources that index, abstract, organize, compile, or digest other sources. Some reference materials and textbooks are considered tertiary sources when their chief purpose is to list, summarize or simply repackage ideas or other information. Tertiary sources may not credited to a particular author.
Most of the time, you should avoid citing any encyclopedias or tertiary sources in your papers. Instead, look at the References and Bibliography sections of scholarly articles and consider using those sources instead. When in doubt about whether a source is appropriate for your assignment, ask your professor or a librarian.
It is important to note that these categories, i.e. secondary and tertiary, are not mutually exclusive. A single item may be primary or secondary (or even tertiary) depending on your research topic and the use you make of that item.
Tertiary sources occasionally have some overlap with other materials. As seen in "Secondary Sources," encyclopedias are sometimes considered secondary sources. Again, remember that you can always check with a librarian or your professor if you need help identifying the type of source you're using.