Longman Dictionary of Contemporary English
Wiktionary
n. (context historiography English) Any document that draws on one or more primary sources and interprets or analyses them; also sources such as newspapers whose accuracy is open to question.
Wikipedia
In scholarship, a secondary source is a document or recording that relates or discusses information originally presented elsewhere. A secondary source contrasts with a primary source, which is an original source of the information being discussed; a primary source can be a person with direct knowledge of a situation, or a document created by such a person. However, as discussed in detail in the section below on classification, how to classify a source is not always an obvious decision.
Secondary sources involve generalization, analysis, synthesis, interpretation, or evaluation of the original information. Primary and secondary are relative terms, and some sources may be classified as primary or secondary, depending on how they are used. A third level, the tertiary source, such as an encyclopedia or dictionary, resembles a secondary source in that it contains analysis, but attempts to provide a broad introductory overview of a topic.
Usage examples of "secondary source".
The Mark II could also operate on electrical power from a secondary source (such as local civilian generator capacity) to maintain long-term readiness in the area defense role.
Since you are not with the Ministry of Information, sir, and knowing that whatever you say will remain confidential, can you give us a secondary source?
There was the violence, hot angry waves rolling in to swamp her and a secondary source, completely different.
The secondary source was sunlight, which penetrated to the interior of the Moon through the hundreds of thousands of huge craters penetrating the lunar crust.