1) Same as virtual host. (http://content.techweb.com/encyclopedia/)
2) A configuration of a World-Wide Web server that appears to clients as an independent server but which is actually running on a computer that is shared by any number of other virtual servers. Each virtual server can be configured as an independent web site, with its own hostname, content, and security settings. The Domain Name System or DNS maps the hostnames of all virtual servers on one physical server to its IP address. The web server software then uses the “Host” header in the HTTP request to determine which virtual server the request was for, and then processes the request using that virtual server's configuration. (foldoc.org)
3) Multiple servers that appear as one server, or one system image, to the operating system or for network administration.