'Uncut pages', or simply 'uncut', traditionally refers to a book which has not been trimmed by rebinding. Prior to the mid-nineteenth century publishers often sold books with a paper or cloth binding intended to be temporary. The purchaser of the book would then make arrangements with a binder to have the book cut and suitably bound, typically in leather. Each time a book is rebound the text block must be trimmed, and multiple rebindings will noticeably reduce the size of the text block and margins of the book.
The term is also often used to describe a book with the pages still attached to the adjacent page at the top or fore edge. This usage is common but technically incorrect, and the preferred term is 'unopened' or from the italian, intonso, literally 'untouched'.
Lastly, uncut is also loosely used to refer to books with deckle or untrimmed edges. Purists will point out that this usage is also incorrect.