Here's an image I pulled off the Internet:
In the single stack, of course, all the rounds are in a single line vertically. In the "double stack," the rounds aren't side-by-side, they are offset vertically and there is slight horizontal overlap. But ... each of the two vertical stacks has each round in that stack making contact with the round above it and the round below it. I would consider that to be the functional definition of a "double" stack. If a magazine is wider than what maintains all the rounds in a single, vertical row but isn't wide enough to allow rounds in each stack to touch the rounds above and below, then IMHO it isn't a single stack (obviously) and it also isn't a double stack. I have facetiously used the term "stack and a half" or "one and a half stack" to describe such magazines, but what would be a more technically accurate description?
"Staggered stack"?
"Staggered single stack"?
"Offset single stack"?