Tertiary source
A source that compiles and summarises secondary literature: encyclopedias, textbooks, handbooks, and most reference works.
Tertiary sources are maps. They are the fastest way to get oriented in an unfamiliar area, find the standard terminology, and identify the names and papers that matter.
They are usually a poor thing to cite. An encyclopedia entry is a summary of a summary, and citing it signals you stopped at the map. Use it to find the primary and secondary sources, then cite those. Wikipedia is the archetype here — genuinely useful for orientation and its reference list, rarely appropriate as evidence.