Garthanos
Arcadian Knight
Well, like most 'skills' in 4e it is not very narrowly defined. I take it to be a general aptitude and interest in/experience at building or creating functional things. That would include constructing buildings and other large engineering works (IE civil engineering) as well as mechanical engineering.
I would consider it to be applicable to any task where a PC is trying to build something, answer some question about a built thing or the construction of a thing, or construct or repair some sort of mechanical contrivance (in a fairly loose way, so I would consider fixing a door would be a reasonable application of Engineering, though a PC with a background in carpentry might be better suited to the work).
I consider it to replace dungeoneering in all of its role of understanding mines and other 'underground structures'. Since many of us also applied it to similar above-ground scenarios as well (for lack of a better alternative) I think that it is almost just a relabeling of that aspect of Dungeoneering. Obviously the "underground nature expert" aspect can be subsumed under Nature itself, which to me is not really a problem (again, Nature indicating a knack for 'natural things' then it seems like a perfectly good thing to apply underground in that role).
I think fortifications and siege engines should be mentioned explicitly in the definition of this I think.
Also i think its building larger scale things .... smaller clock work construction if there is a heroic skill which covers it is usually a nod to thievery.