Ryan Lascano, in a not-so-recent article for Arrows & Icons Magazine, argues that environmental graphic design (EGD) is design within the environment. Taking his cue from SEGD, the Society for Environmental Graphic Design, he posits that its role is to "tell stories or communicate messages and information through the built environment." While this is as textbook a definition as you are likely to find, there is insufficient detail here to distinguish EGD from vanilla graphic design—now called visual communication. And certainly the topic of what is EGD, is still unresolved.
The problem Lascano faces is that in attempting to define EGD as a distinct multidisciplinary field, he actually ends up positioning it as nothing more than a meeting point in the grand continuum of designer disciplines.
Does the environmental graphic designer require any specialist skills beyond those present within the broader design community? The graphic attached to his article asserts that EGD combines "Signage & Wayfinding; Interpretation and Placemaking."
Certainly wayfinding/-showing is a specialised skill, but is in no way peculiar to EGD—we perform some sort of wayfinding every day.
The same can be said of the terms of Interpretation and Placemaking, the latter of which seems to be a hit and miss affair, while the former is true of all design disciplines. Accordingly, EGD, as currently defined, is a mix of several, relatively well defined design disciplines—namely graphic design, interior architecture, industrial design and wayfinding/-showing. To that end, it is arguable whether EGD is a discipline in its own right.
In encapsulating all design disciplines it ends up being none of them—and so cannot be defined beyond the simplistic "design in the built environment".
However, if we consider it as part of wayfinding or user experience (UX), it becomes clear that EGD is part of the toolset that designers of built environments have to use. Further evidence of this is given in Lascano's article when he argues that the importance of EGD can be found in its ability to guide people through new places.
Thus EGD can partly be defined as spatial problem solving. But again, this is not specific to EGD—it is from the fields of wayfinding/-showing and user experience.