A zombie server is a physical server that is running but has no external communications or visibility and contributes no compute resources; essentially, it consumes electricity but serves no useful purpose.
Zombie servers are often created because user-requested applications end up getting no use or almost no use (typically defined as under six percent). Other causes include redundant or legacy applications and services that have been replaced. An estimated one in three servers in North America falls into the "undead" category.
According to a study conducted by the consulting firm Anthesis Group and Jonathan Koomey, a research fellow at Stanford University, there are approximately 3.6 million zombie servers in the United States; worldwide, the total could be as high as 10 million. Theoretically, four gigawatts of power could be saved by killing zombie servers. Based on calculations by TSO Logic, a company with 1000 servers could net savings of $300,000. AOL's five-year project to purge its sites of zombie servers netted the organization $10 million and in just one year resulted in a 35 percent reduction in its carbon footprint.
To reduce the server sprawl associated with zombie servers, multiple low-utilization servers may be combined into single virtual servers. Servers that are entirely unused can sometimes be repurposed. Otherwise, IT should just kill the zombie -- by pulling the plug.
Zombie servers are sometimes referred to as comatose servers.
See a brief video about finding and fixing zombie servers: