Virtual domain.
A domain that exists as a software entity on a server as contrasted with a domain that actually is comprised of a dedicated hardware location. A virtual domain has a numeric IP address and can include websites, mail servers, FTP sites or any other resource type.

Virtual machine.
The environment created by a Java-enabled Web browser that Java applets run within. A software simulation of another computer. A virtual machine is useful for testing software on large computers such as mainframes.

Virtual memory.
A use of disk drive storage that simulates RAM. Some operating systems (not DOS) borrow parts of the disk drive and swap out massive chunks of memory to a file on disk—a swap file. That way true memory (RAM) is made available for programs that need it. The memory saved on disk can be put back into real memory when it’s needed later. 

Virtual reality (VR).
A general term for any computer-generated world which use advanced  techniques to make this world seem ‘real’ to the user. Typically, the user will wear a helmet containing two tiny screens, one for each eye, whose displays combine to form a 3D image of the play area; interaction with people and objects in the virtual world is via a glove-like device which has sensors to detect the wearer’s hand movements and translate them into movements of a graphically displayed hand.

Virus.
A computer virus is a self-replicating program containing code that explicitly copies itself and that can “infect” other programs by modifying them or their environment such that a call to an infected program implies a call to an evolved copy of the virus. Computer viruses are never “naturally occurring”; they are always man made. Once created and released, however, their spread is not directly under human control.

Vulnerability.
Any weakness, administrative process or act, or physical exposure that makes a computer susceptible to exploit by a threat.