At http://stackexchange.com/mediakit you can see two online advertising plans, one of which says:
ROS global only
What does the 3rd line "ROS global only" mean?
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It stands for Run Of Site.
This means if you go for ROS, your advertisement in the form of banner, link, image, media or whatever will run on the targeted site. The page of that site could be any.
However, a little difference you must understand is this. The source is Digital Marketing Glossary.
Run of site or ROS is an ad buying option by which ad appears and rotates on any pages of the chosen site. In the case of a run of site buy, ads are typically delivered on less valuable pages, sections and placements. Therefore run of site inventory is sold at lower rates than premium inventory. Run of site inventories are bought by advertisers, but also by some ad networks which bulk buy for reselling at CPM, CPC or CPA. Retargeting networks are typically big buyers of ROS.