folio

folio - n - A large sheet of paper folded once to make two leaves, or four pages, of a book or manuscript; a volume having pages of the largest size (about 15 inches), formerly made from such a sheet.

Many of Shakespeare's plays were published in smaller (quarto) editions during his lifetime, but these were what today would be considered pirated editions. Moreover, Shakespeare did not have any say in these publications, so their quality is erratic to say the least. His complete plays were first published in 1623, in what is now known as the First Folio; others were to follow. These different editions are why there were - and still are - Shakespeare scholars; the burning question continues to be, "What did Shakespeare actually write, and what was finally performed. To give just one example out of many possible, there's a soliloquy in Hamlet that begins, "Oh, that this too too solid flesh would melt. . ." One edition I've seen renders that as, "Oh, that this too too sullied flesh would melt. . ." The story continues, offering yet another proof that Shakespeare was not just for his time, but for the ages.