I lived 5 and a half years in North SnoCo/Island Co in Western WA and currently live almost as far east as you can get before going into Idaho. I moved here almost three years ago.

In both places: Western WA = West of the Cascade Crest / any of the counties west of the Cascades. Eastern WA = All of the remaining counties east of the Cascades.

A subregion of Western WA could be the Puget Sound Region or another example, SW Washington which aligns more with Portland. See Clark County as an example.

Central WA = Central Washington University in Ellensburg - nice little town. The Central WA area is more like a subset or subregion of Eastern Washington as a whole. If you really want to know what counties would consider themselves central, here is a map: https://en.wikipedia.org/wiki/Central_Washington#/media/File:Central_Washington.svg

For the counties that are light pink in that link... I don't really consider Kicklitat (despite being in the center of the state), Benton / Franklin (Tri Cities), or Adams (the county that looks like a backwards Idaho or Utah) to be Central WA.

The main difference is Eastern Washingtonians call it the "East Side" whereas in Western WA, the East Side refers to the eastern Suburbs of King County.