Last Thursdayism (alternately Last Tuesdayism or Last Wednesdayism) is the idea that the universe was created last Thursday, but with the physical appearance of being billions of years old. It’s also a counter to the creationism theory. Under Last Thursdayism, books, fossils, light already on the way from distant stars, and literally everything (including your memories of the time before last Thursday) were all formed at the time of creation (last Thursday) in a state such that they appear much older.
Last Thursdayism functions both as a philosophical point on how our observations may not match with “reality” and a reductio ad absurdum of the young-Earth creationist idea of the omphalos hypothesis: if the world was created 6,000 years ago with the appearance of being made billions of years ago, what is there to stop us from claiming it was made Last Thursday?
The debate on whether Last Thursdayism is true has raged on ever since the creation of the universe last Thursday.