What's The Name For The Blank Spaces Between Stamps?

'Vertical Spacing X, Horizontal Spacing X ???

With this specific case, where the stamp is imperforate, then there is no specific term to refer to that space. You don't need a specific term to refer to it, but "spacing" works well as a descriptive general term. If the stamp is perforated, there is still no specific term to refer to the total space separating the stamps, but once perforated the space becomes the stamp's margin.

Well, this is interesting. I just looked at L.N. Williams, "Fundamentals of Philately," (State College PA: APA 1990) and he defines "gutter" as "The spaces between the design of one unit and that of the next (whether the unit be a stamp, a pane or a sheet) but only while the units are unsevered. Thus it is possible, in the case of some printed-sheet formations, to have 'Stamp Gutters,' 'Pane Gutters,' and even, 'Sheet Gutters,' but 'Printed Sheet Gutters' cannot exist. Often the term 'Gutter,' unqualified, impliles only the spaces between stamps normally and closely set. Compare and distinguish 'Margins.'" P. 142. Sorry I didn't check there first.

Navalon, I think you just proved that a picture is worth a thousand words, or perhaps a couple of dozen posts.

Margin is the correct term. When looking at a single stamp, say a U.S. #1, an imperforate sitting Helvetia or a Penny Black, the value depends on whether there are four clear "margins". The other alternative, to avoid confusion, is to qualify your term appropriately with text, usually is in high abundance in this age of terabyte storage. Something like "margin between stamps" might suffice to be absolutely unambiguous.

"Gutter" it is according to the foremost authorities! I sprung for $5.62 including shipping and bought a copy of Mackay's "Philatelic Terms Illustrated" published by Stanley Gibbons. The "foremost authorities" (apologies to Prof. Irwin Corey) agree that the blank spaces separating stamps are called "gutters". See L.N.Williams quoted previously and Mackay ("Gutter: The space left between stamps to allow them to be separated or perforated.") Mackay went on to define the blank spaces between panes, "usually of stamp format," as "interpane gutters."