When used in reference to online advertising, one server assigning an ad-serving function to another server, often operated by a third company operating on behalf of an agency.
For instance, a Web publisher's ad management server might issue a redirect to the browser or client which points to an Agency Ad Server (AAS) hired by an advertiser to distribute its ads to a target audience across a broad list of sites. There is no limit to the number of redirects that can come into play before the delivery of an actual ad. The agency ad server in turn may redirect the browser to a Rich Media Vendor (RMV) or Digital Video ad server.
Re-directs produce latency! This is especially true when they are client side redirects which is the case in most online advertising today. Server side redirects limit latency but also limit the ability to persist the user’s identity when those redirects cross domains.