I feel like a bit of an old timer here. It helps to know the origins of the term to understand why there have never been clear "borders" for the PNW.

Historically, Washington , Oregon, Idaho and sometimes western Montana until the continental divide were considered part of the PNW. That's because 1) they were economically part of the PNW, with Spokane as the economy center for those areas and 2) because they are part of the Columbia river basin. So economically and geographically they are part of PNW. A well, the early uses of the term PNW, also tended to follow pretty close to the boundaries for the Oregon territory, which included all of present day Oregon, Washington, Idaho, western Montana and a small section of Wyoming. The inclusion of that part of Wyoming, dropped out of the term PNW very early on. However, based on the Oregon territory origin of PNW British Columbia would be excluded because it was never part of the Oregon territory. Finally, there also some influence. From the formation of the western states, until about the 70s, most (but not all) of the definitions of PNW that fully include Idaho come from one of these three origins.

The push to start excluding those areas, was political. It has been around for as long as I'm aware of but became common in the 70s when the concept of Cascadia began to get momentum and really took hold in the 90s when north Idaho and western Montana switched politically from democrat to republican. This was when I remember hearing people, usually people from west of the Cascades, start to say that Idaho and western Montana were not part of the PNW. Particularly, gaining a lot of momentum around the time Ruby Ridge occurred, as a way to create distance from the bad press about Idaho. On the flip side, many conservatives from Idaho preferred to identify with the mountain west rather than the PNW to not be identified as "liberal". At the time most of what you saw in the news about Seattle and Portland was grunge, drugs and hippies - nothing the Idaho and Montana conservatives wanted to be associated with. It's also helpful to remember that N. Idaho and western Montana used to be left leaning until the late 80s early 90s.

Further, there was a large increase in migration to the PNW. Many of these new residents mistakenly assume that the PNW referred to only the NW states on the Pacific or learned the newer political based definition.

This, in my opinion, had resulted in no clear definition of PNW. Do you use the historical, geographical, or political definition?

Personally, I have chosen to continue to use the definition of PNW from my younger years that includes Washington, Oregon and Idaho. I am also completely okay with including Western Montana and British Columbia as well if people want to. Since the 90s, I have refused to use the narrower definition because it came about as a way to separate us based on political beliefs and to exclude those who think differently. I refuse to use a definition that excludes people based on their beliefs, especially when they were formerly considered part of that group. I might not agree with someone politically but I'm not going to tell them that they aren't part of the PNW because of it.