High-railers

Hi-rail means that engines, cars, buildings, scenery are scale sized but running on 3 rail track.  The term for hi rail came from the fact that the old tubular track is much higher then scale track hence the name. Although all 3 rail track is over sized. You can do as much in hi rail or as little that suits your taste in model railroading. There is also 3 rail scale (3RS) segment that build layouts that are truly O scale except for the track which of course is 3 rail. The locos and cars also have scale couplers. 3RS and hi rail are close to being the same except it just depends how far you are willing to go.

I have thought it was OK to have semi-scale trains, but with scenics that were fully developed  and not "toy like".

While we are on the subject, what is 2-rail "O" running on bare plywood called?

I'm in line with Joe (though I have seen David's definition before); you try to be as prototypical/life like as possible whether scale or semi-scale.

Redball 

O gauge refers to the track gauge.  It preceded the development of scale model trains.  O gauge track is inherently not scale unless you scale the models to 1/43.  There is a branch of the hobby that does use true 1/48 scale models called Proto 48.  These models do not run on O-Gauge track.

Plywood Central...

Seriously, even though 2-rail O is running on bare plywood (assuming scale track, flanges and couplers) is still considered O Scale.

And don't forget there is also a HiRail contingent in S Scale.

Rusty