Run of site or ROS is an ad buying option by which an ad appears and rotates on any pages of the choosen 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.
See also run of network, aka RON.
An example of ROS pricing on Adage.com :