Acronym for Global Server Load Balancing
Load balancing for multiple PoPs/sites, trying to give users closer access to the content. Usually done by a NameServer that also knows about the load on the PoPs, so that it can swing traffic to the one which is loaded least, similar to an L4 switch.
This is different from RoundRobin DNS using multiple A ResourceRecords. It tries to be intelligent about sending connections to locations with low load. Akamai, f.ex., uses a combination of GSLB with DNS, ping, traceroute, and BGP.