Is prehistoric a noun?
The 'Prehistoric' is a noun, a proper name, and is capitalized. However, 'prehistoric' is an adjective, and is not capitalized.
"We journeyed back to The Prehistoric, where we saw prehistoric life."
The noun 'encyclopedia' is a common noun, a general word for any encyclopedia of any kind. A proper noun is the name or title of a specific person, place, or thing. A proper noun for the common noun 'encyclopedia' is the name of an encyclopedia, for example, Encyclopedia Britannica or The Complete Illustrated Encyclopedia Of Dinosaurs & Prehistoric Creatures by Dougal Dixon.
Irving Rouse has written: 'Introduction to prehistory' -- subject(s): Anthropology, Prehistoric, Man, Prehistoric, Prehistoric Anthropology, Prehistoric Man 'Migrations in prehistory' -- subject(s): Anthropology, Prehistoric, Man, Man, Prehistoric, Migration, Prehistoric Anthropology, Prehistoric Man