A "One Pager" is meant to be a high-level-overview on a topic, generally printed on ONE side of paper.
These are frequently used in government when an agency is trying to pass a bill or convince others on department policy, like this:
Here's a One Pager that tries to convey different thinking errors:
Here's a One Pager used in AltMBA for potential students to give to their employer:
I'm a big fan of One Pagers myself, as they can now be used for social media images, website assets, and used in all your marketing materials!