a book (or manuscript) consisting of large sheets of paper folded in the middle to make two leaves or four pages; "the first folio of Shakespeare's plays"
a tentative theory about the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena; "a scientific hypothesis that survives experimental testing becomes a scientific theory"; "he proposed a fresh theory of alkalis that later was accepted in chemical practices"
a proposal intended to explain certain facts or observations