Available disk space that is not allocated to any partition, logical drive, or volume. The type of object you can create on unallocated space depends on the disk type (basic or dynamic). For basic disks, you can use unallocated space outside partitions to create primary or extended partitions. You can use free space inside an extended partition to create a logical drive. For dynamic disks, you can use unallocated space to create dynamic volumes.
See also Free Space.