When is the most commonly accepted date that delineates 'prehistory' from 'ancient' history? What is meant by 'prehistory'?

Prehistory is the time before the existence of "Written Records" and the human ability to write e.g. the era of Dinosaurs.

Prehistory is the period before written history became available to assist our understanding of the past. The term was introduced into English by Daniel Wilson (1816–1892), President of Toronto University College, in 1851.

https://www.newworldencyclopedia.org/entry/Prehistory

Ancient history begins with the human development of writing.

Ancient History is considered to be the first period of the History of Humanity, preceded by pre-history and followed by the Middle Ages. According to tradition, the invention of writing marks the beginning of Ancient History, as it was a very important moment in history that allowed humans to develop a more advanced form of communication. Likewise, the end of Ancient History is dated around the year 476 AD with the fall of the Western Roman Empire.

https://ancientcivilizationsworld.com/what-is-ancient-history/

Since written language developed at different times in different places, there is no one universal date for the beginning of Ancient History, although many European historians (and those in areas colonised by Europeans) tend to use around 2600-2700 BCE, with Egyptian and Sumerian records emerging. The end of Ancient History, as shown above, is traditionally linked to the Roman Empire, a very Eurocentric demarcation indeed!

https://en.wikipedia.org/wiki/List_of_languages_by_first_written_account