1. A contiguous section of a disk drive that is treated by the operating system as a physical drive. Thus, one disk drive can have several drive letters assigned to it. From I-gloss

2. A section of the storage area of a bard disk created for organizational purposes or to separate different operating systems. A partition is created during initial preparation of the hard disk, before the disk is formatted. In MS-DOS, every hard disk has at least one DOS partition (Version of MS-DOS before version 4.0 required a separate partition for each I'm of disk space). Each partition is treated by DOS as though it were a separate drive. Users may partition their drives to separate one operating system from another, but utility programs such as MultiDisk, are available that let you create several system partitions. See directory and subdirectory. From QUECID

3. A contiguous section of blocks on your hard disk that is treated like a completely separate disk by most operating systems. From Linux Guide @FirstLinux