The doghouse is the portion of a full-frame car which is forward of the firewall (or bulkhead.) It includes the fenders, inner fenders or fender aprons, hood, and last in order but first in importance, the radiator support which aligns all of this. A doghouse is similar to a front clip, but on a full-frame car, the entire assembly can be unbolted from the frame without any cutting. Like a clip, the doghouse can also include a header panel (which holds the headlights and the grill), and a valence or spoiler.
The doghouse gets its name from the simple fact that it made a dandy lurking place for the proverbial junkyard dog; It is enclosed by the radiator support, firewall, and fenders, but can be entered or exited through the wheel wells, or possibly through an opening for the transmission in the firewall.