"A virtual appliance is a virtual machine image designed to run on a virtualization platform (e.g., VirtualBox, Xen, VMware Workstation, Parallels Workstation)."
http://en.wikipedia.org/wiki/Virtual_appliance
So a Tiki appliance is Tiki designed to run on a virtualization platform.
Some appliance providers:
http://www.jumpbox.com/app/tikiwiki
http://www.standingcloud.com/network/tikiwiki
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