Of or pertaining to a period before written history begins; as, the prehistoric ages; prehistoric man.

Belonging to a time millions of years ago, before history was written down

Any time before recorded history began. Generally before the advent of writing in any given area.

Belonging to an early period and a museum. Antedating the art and practice of perpetuating falsehood. He lived in a period prehistoric, When all was absurd and phantasmagoric. Born later, when Clio, celestial recorded, Set down great events in succession and order, He surely had seen nothing droll or fortuitous In anything here but the lies that she threw at us. Orpheus Bowen

Any period of which there is no contemporary documentation. It is the main segment of human past.

This literally means anything that happened before humans had developed 'history' or a way of passing down through the generations an idea of what had happened in the past. It is difficult to say that prehistory starts at a particular date, as the term is used by different people (historians, archaeologists and palaeontologists) to mean different things.

no longer fashionable; "my mother has these prehistoric ideas about proper clothes"