What's "scan column" and why do we name it like this? Why not just call 'left column' or 'right column'?
It sounds to me like a column the user scans with her sight, like for example a navigation menu.
Jakob Nielsen said years ago, in one of his alertboxes about how users read in the web, that users scan page content as opposed to fully reading it.
Also, in forms it is recommended to left-align the labels because if they are right-aligned the users cannot scan them. In fact, not being able to scan it is what makes columns of centered text so horrid.
It refers to rapidly browsing with the sight.
I think it's because they are columns that are mainly used to scan for information or navigation. They don't provide content.
Example: this usability site has a picture of which areas are mainly read in a site.
But I don't think that those terms are that popular. Search for "left scan column" in google will only give you 217 results...
As to why they are named scan columns, Web Style Guide Online explains:
"... much as scan columns have been used in print publications for a century or more."
after stating that (scan columns)
"organize navigation links and other peripheral page elements."