Definition of a tertiary source according to a journal:

A tertiary source is a type of publication that synthesizes information from primary and secondary sources to provide an overview or analysis of a topic. Tertiary sources often include encyclopedias, dictionaries, textbooks, and other reference materials that distill and summarize information for a general audience. They are useful for gaining a broad understanding of a subject and for finding additional sources for further research.

A tertiary source provides an overview of a topic and is written for a general audience.
Examples of tertiary sources include:
- encyclopedias
- textbooks
- dictionaries
- almanacs
- guidebooks
- popular magazines