That's exactly what it was.

Back in the colonial era, the Calvert family (who controlled Maryland) and the Penn family (who controlled Pennsylvania and Delaware) agreed on the principles for defining the boundaries of those colonies. To make a long story short (and the Wikipedia articles on "The Wedge (border)" and "Twelve-Mile Circle" do a fairly good job), the northern boundary of Delaware was defined as an arc of a twelve-mile circle centered on a particular building in New Castle, Delaware. The Mason-Dixon Line was surveyed from a particular point, westward (to form the PA-MD border) and also southward (to form the DE-MD border). But that point of origin turned out to be outside of the circle, which left a roughly-triangular parcel of land (the "Wedge") that was south of the Mason-Dixon Line (and therefore arguably not part of Pennsylvania), east of the north-south surveyed line (and therefore definitely not part of Maryland), and outside the Circle (and therefore arguably not part of Delaware). This ambiguous state of affairs prevailed until PA and DE agreed that it was part of Delaware, and certain other details of Delaware's boundaries were clarified.

TL;DR: You can construct Delaware with a ruler and compass, but it's harder than people assumed.

Also, the Twelve-Mile Circle means that that within the Circle, the entire Delaware River is part of Delaware and not New Jersey (as opposed to the boundary being at the river's thalweg which is a more typical situation). This has implications when people try to build piers into that part of the river.