What exactly constitutes the downtown of Portland, OR is a matter of much debate. While it is generally considered that the Willamette River is the eastern boundary of downtown, the other borders are somewhat more hazy.
The most common definition of downtown is probably Fareless Square, or the area where Tri-Met busses can be ridden for free. This area has borders of the Willamette River on the east, NW Irving on the North, and I-405 on the South and West.
However, on the west side, many people consider everything up to the Portland Hills as being part of downtown, including Civic Stadium (or whatever it thinks it is called these days) and Trendy Third. While going all the way out to Twenty Third and Northrup is probably not downtown, neither would I say that downtown stops at I-405. On the south and north we have a similiar situation, I would definitly consider the Pearl District and the area to the north of it for a few blocks as part of downtown.
I guess the best answer is, if you are on the MAX and it is making stops every two blocks for no good reason, you are probably downtown. | 
