See the definition in the first post by Rich.
...3-Rail Scale is an attitude toward the O gauge hobby that derives satisfaction and fun from using the prototype as its guide. The intent in 3-Rail Scale is to continually strive for as much prototype realism as possible within the limitations of time, talent, and available space.
More than any other single thing, the desire for realism in miniature distinguishes 3-Rail Scale from other segments of the 3-rail hobby.
