Secondary Source
A work that describes, interprets, or synthesizes primary sources, such as a review article, textbook, or commentary.
A secondary source discusses, interprets, or synthesizes information originally reported elsewhere. Review articles, textbooks, meta-analyses, biographies, and commentaries are typical examples: their authors did not produce the original evidence but analyze or summarize the work of those who did. The classification is relative to your question; a review article becomes a primary source if your study is about how reviews are written.
Secondary sources are valuable early in a thesis, when reviews and handbooks help you map a field quickly and find the primary literature worth reading. The risk is citing them in place of the originals: summaries can be outdated, selective, or simply wrong, and copying a citation from a secondary source without reading the original propagates errors. Use secondary sources for orientation and synthesis, then verify and cite the primary evidence for specific claims.
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