VMDK (short for Virtual Machine Disk) is a file format that describes containers for virtual hard disk drives to be used in virtual machines like VMware Workstation or VirtualBox. Initially developed by VMware for its proprietary virtual appliance products, VMDK became an open format with revision 5.0 in 2011, and is one of the disk formats used inside the Open Virtualization Format for virtual appliances. The maximum VMDK size is generally 2TB for most applications, but in September 2013, VMware vSphere 5.5 introduced 62TB VMDK capacity.