A Gutter is the space in between printed postage stamps. When stamps are printed on large sheets of paper that will be guillotined into smaller sheets along the gutter it will not exist on the finished sheet of stamps. Some sheets are specifically designed where two panes of stamps are separated by a gutter still in the finished sheet and gutters may, or may not, have some printing in the gutter.
A number of different gutter based configurations exist: Gutter pairs are two stamps separated by a gutter. Gutter block is a block of, at least, four stamps where both the vertical and horizontal pairs are separated by a gutter. Since perforation of stamps in the norm, the gutter between stamps are often the same size as a postage stamp.