Helpful Explanation of “Margin”

Over twenty years ago I read (and reviewed somewhere) a really awful book by a political theorist named Philip Green called The Pursuit of Inequality. In it Green quoted an economist speaking of choices on the margin and responded with exclamation marks that “Those differences aren’t marginal! They’re really big!” (Apologies for not having the book at hand for the exact quote.) He had no idea what an economist means by “marginal.” Don Boudreaux at offers a little primer on the topic.