Reference sources like encyclopedias, dictionaries, almanacs contain concise, factual entries often written by different experts on the topic. They provide a brief definition or overview of a larger topic as well as key concepts, terms, or dates. Traditional encyclopedias and collaborative encyclopedias, like Wikipedia, are good places to find background information, understand basic concepts, and verify facts.
Encyclopedias and dictionaries typically refer you to other sources of information; they are often called reference sources. Reference sources typically aren't cited in academic scholarship, but they are excellent places to do background research.
