partition 
A portion of a hard disk that functions as a separate drive. A single hard disk can be divided into several partitions, each of which has its own volume name (such as D:, E:, F:, and so on). This is done to make the drive more efficient--as the computer can search smaller sections for a specific file or program--or to set aside a portion of the hard drive for a particular purpose, such as loading a secondary operating system.